feat(Model): support credential-get hook tool in both model and harness

[IronCore864]

Added Model.get_cloud_spec which uses the credential-get hook tool to get details of the cloud where the model is deployed.

Changes are made to both ops.Model and Harness.

Closes #186.

[IronCore864]

Sample usage:

class MyVMCharm(ops.CharmBase):
    """Charm the application."""

    def __init__(self, framework: ops.Framework):
        super().__init__(framework)
        framework.observe(self.on.start, self._on_start)

    def _on_start(self, event: ops.StartEvent):
        try:
            cloud_spec = self.model.get_cloud_spec()
            print(CloudSpec: {cloud_spec!r}')
        except ops.ModelError as e:
            logger.warning(
                f'credential-get hook tool does not work on containerized environment: {e!r}\n')

        self.unit.status = ops.ActiveStatus()

Output:

unit-microsample-5: 09:32:40 DEBUG unit.microsample/5.juju-log Emitting Juju event start.
unit-microsample-5: 09:32:40 DEBUG unit.microsample/5.start CloudSpec: CloudSpec(type='lxd', name='localhost', region='localhost', endpoint='https://10.76.251.1:8443', is_controller_cloud=None, credential={'auth-type': 'certificate', 'attrs': {'client-cert': '-----BEGIN CERTIFICATE-----\nMIICAzCCAYmgAwIBAgIRALRDwDXMD/AjHCTB0JHurvYwCgYIKoZIzj0EAwMwLzEM\nMAoGA1UEChMDTFhEMR8wHQYDVQQDDBZ1YnVudHVAaXAtMTcyLTMxLTQyLTIyMB4X\nDTI0MDMxMzA1MjIzN1oXDTM0MDMxMTA1MjIzN1owLzEMMAoGA1UEChMDTFhEMR8w\nHQYDVQQDDBZ1YnVudHVAaXAtMTcyLTMxLTQyLTIyMHYwEAYHKoZIzj0CAQYFK4EE\nACIDYgAEpJG0aeaCkFoJ4+CDwiRdBafrA5cYcrRljIfOu9pNh5iKAttV2LtZ5L39\nCun13Lp531GnL1LKdmJ80pCdI4PPdRPTOXsAQTE5pFdcHuRbVSzqVenIniK3gb7g\n7EtaGVP8o2kwZzAOBgNVHQ8BAf8EBAMCBaAwEwYDVR0lBAwwCgYIKwYBBQUHAwIw\nDAYDVR0TAQH/BAIwADAyBgNVHREEKzApgg9pcC0xNzItMzEtNDItMjKHBH8AAAGH\nEAAAAAAAAAAAAAAAAAAAAAEwCgYIKoZIzj0EAwMDaAAwZQIxALPdVw+tc5njeRD7\nL4WxmcTEiw5N67+nBfDVRfo5LheWcBGey1fUufefdLQBLlLDhQIwOy//D84EMRdU\nR3xJb39jZZ9JTX45qugTb8cd82qvOvEeLYwF52bh8pdgaDchRUdV\n-----END CERTIFICATE-----\n', 'client-key': '-----BEGIN EC PRIVATE KEY-----\nMIGkAgEBBDAa9kwOCVZXdCghEexLdCSmSyaXdWWCD939byLJ+IJhzV/dLlEAh1t5\nJ+XK/F8khf6gBwYFK4EEACKhZANiAASkkbRp5oKQWgnj4IPCJF0Fp+sDlxhytGWM\nh8672k2HmIoC21XYu1nkvf0K6fXcunnfUacvUsp2YnzSkJ0jg891E9M5ewBBMTmk\nV1we5FtVLOpV6cieIreBvuDsS1oZU/w=\n-----END EC PRIVATE KEY-----\n', 'server-cert': '-----BEGIN CERTIFICATE-----\nMIIB/zCCAYWgAwIBAgIRAMRwEkdwDlBEk9aakWv7Q5cwCgYIKoZIzj0EAwMwLTEM\nMAoGA1UEChMDTFhEMR0wGwYDVQQDDBRyb290QGlwLTE3Mi0zMS00Mi0yMjAeFw0y\nNDAzMDQwMTMzMTBaFw0zNDAzMDIwMTMzMTBaMC0xDDAKBgNVBAoTA0xYRDEdMBsG\nA1UEAwwUcm9vdEBpcC0xNzItMzEtNDItMjIwdjAQBgcqhkjOPQIBBgUrgQQAIgNi\nAARMwnKekEznWQTKJYYygp3FJItJha/k6pCUejhuH1q7kO+cfhfZHI+S7xrWRv9q\nH4L3e/xQ+1WriclGStmgn1kRg1GEB2TbCc3D2QJtS6a9yIsTkU3kZxWGp8MDnqN9\n0EajaTBnMA4GA1UdDwEB/wQEAwIFoDATBgNVHSUEDDAKBggrBgEFBQcDATAMBgNV\nHRMBAf8EAjAAMDIGA1UdEQQrMCmCD2lwLTE3Mi0zMS00Mi0yMocEfwAAAYcQAAAA\nAAAAAAAAAAAAAAAAATAKBggqhkjOPQQDAwNoADBlAjAU07TtAgIF746wMa53TJP/\nyU8tMTVl2aUi/uhDCo75PeUAP/G9+Twximprx7VNCGwCMQDHJTOYZLqiLpTq/izR\nO5txgVv3U1CcrBvfaODD4lqrQxr9Qt/uJLhveVPdRLKTvag=\n-----END CERTIFICATE-----\n'}}, identity_endpoint=None, storage_endpoint=None, ca_certificates=None, skip_tls_verify=None)

Deploy in K8s:

unit-microsample-0: 09:37:40 INFO juju.worker.uniter found queued "start" hook
unit-microsample-0: 09:37:41 INFO unit.microsample/0.juju-log Running legacy hooks/start.
unit-microsample-0: 09:37:41 WARNING unit.microsample/0.juju-log credential-get hook tool does not work on containerized environment: ModelError('ERROR cannot access cloud credentials: credential-get on a "caas" model not supported\n')

Harness sample usage in docstring.

[benhoyt]

Thanks, this is a good start! I haven't gone over it with a fine-toothed comb yet, but just a few initial comments. @tonyandrewmeyer will review as well.

[tonyandrewmeyer]

Looking great! A few little suggestions, particularly around documentation.

[IronCore864]

All comments resolved, please take another look.

[benhoyt]

Good progress here, thanks. A whole bunch of comments, mostly very minor, but also a bit of fixes to do with the field names and types from the Juju structs.

[tonyandrewmeyer]

Looking great! Just a couple of small doc requests.

[benhoyt]

Thanks. This is very close now! Just a few minor suggestions to improve the dataclass fields and defaults.

[IronCore864]

Refreshed according to latest review, please take another look :)

[tonyandrewmeyer]

Looks great, thanks! Nothing from me, other than the Optional adjustment that Ben mentioned.

[IronCore864]

CloudCredential and CloudSpec typing fixed @benhoyt.

[benhoyt]

Thanks, approved! Let's please just do two things before you merge: 1) add a line to CHANGES.md describing the new feature, and 2) describe the new feature briefly in the PR description.