Ensure logo updated when source file changes

[gavinbunney]

Compute and store the hash of the logo alongside the path to ensure any changes to the source file are reflected on Okta. Without this change, the logo is not updated as the path attribute hasn't changed.

Fixes: #512

Validated:

1. New logo set
2. Change logo path
3. Change logo image

Example output

  ~ resource "okta_app_bookmark" "confluence" {
        id                  = "XXXXXXXXXXX"
      ~ logo                = "./../../assets/app-confluence.png (320385e0df8be4557e73dfdb2e58ca53da25ecdf8a9b85f0be4d7f61e9281681)" -> "./../../assets/app-confluence.png (6904d1f02905f4365311ec9d9e161eda995001dc122150f466922ef41f9022da)"
        name                = "bookmark"
        # (10 unchanged attributes hidden)
    }