Disk encryption and backup disk encryption

[shemau]

Description

#49 Clarification around disk encryption and backup disk encryption requirements.

Adding documentation to the README.md and improving the descriptions of the key_protect_key_crn and backup_encryption_key_crn.

Include some validation on the backup_encryption_key_crn. This is unlikely to catch much. It does not catch when the value is null and key_protect_key_crn is substituted. It does not catch when the key is created as part of the root module, since the key is not known until apply time.

We can not validate this inside the module, because if the module validates and fails, it becomes impossible to run plan or destroy against the state file. These actions would be required to destroy and recreate the key and/or KP instance.

Types of changes in this PR

No release required

• Examples or tests (addition or updates of examples or tests)
• Documentation update
• CI-related update (pipeline, etc.)
• Other changes that don't affect Terraform code

Release required

• Bug fix (patch release (x.x.X): Change that fixes an issue and is compatible with earlier versions)
• New feature (minor release (x.X.x): Change that adds functionality and is compatible with earlier versions)
• Breaking change (major release (X.x.x): Change that is likely incompatible with previous versions)

Release notes content

Add validation to backup_encryption_key_crn to validate backup keys are from us-south/us-east/eu-de.

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Checklist for reviewers

• If relevant, a test for the change is included or updated with this PR.
• If relevant, documentation for the change is included or updated with this PR.

Merge actions for mergers

• Merge by using "Squash and merge".

• Use a relevant conventional commit message that is based on the PR contents and any release notes provided by the PR author.

  The commit message determines whether a new version of the module is needed, and if so, which semver increment to use (major, minor, or patch).

[ocofaigh]

@shemau I have taken your updates and mostly added them to #186
Please take a look. I'm going to close this PR