feat: add support for multi-account

[t1agob]

Description

This PR introduces support for multi-account configuration. With this PR admins will be able to specify multiple PagerDuty accounts in the same Backstage instance. Support was added across all the components of the plugin:

• Backend: All API routes exposed take account as an optional parameter so you can create and query information from different PagerDuty Accounts. If admins leverage the Entity Mapping feature, the account is now persisted into the plugin database.
• Scaffolder Actions: When the scaffolder action is used on a template it now lists Escalation Policies from all accounts configured so the user can easily select the one they want. The Scaffolder action also outputs the account so it can be replaced in the app-config.yaml annotations.
• Entity Processor: If an entity mapping stored in the plugin database contains the account property, the processor will add an annotation to the entity with its information.

This PR solves an existing problem for many large organizations that have several PagerDuty accounts for segregation purposes or that result from company acquisitions.

Issue number: #96

Type of change

• New feature (non-breaking change which adds functionality)
• Fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have performed a self-review of this change
• Changes have been tested
• Changes are documented
• Changes generate no new warnings
• PR title follows conventional commit semantics

If this is a breaking change 👇

• I have documented the migration process
• I have implemented necessary warnings (if it can live side by side)

Acknowledgement

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