r/attestation_provider: conditionally building the Authorizer as needed and refactoring the Resource

[tombuildsstuff]

Part of reviewing #21509 has identified a number of issues with the PR submitted to add support for this, #20972.

This PR fixes both #21508 - and works to fix some design issues which should have been caught at both implementation and review phase - namely where:

1. Rather than pre-creating this Authorizer (and failing in Environments where this isn't defined) - we can use the Data Plane URI that's returned from the Resource Manager API to build this up depending on the Environment
2. Rather than using a List for this and trying to ensure all the environment_type keys are consistent, these should have been exposed as individual fields for each Policy Type - whilst changing that prior to 4.0 would be a breaking change, since we can read these values back from the API (see the next item) this would need to become a Set rather than a List.
3. The Read method doesn't retrieve these values at all, so we'll never flag there's a diff for these values - although they can be retrieved via the Data Plane endpoint (by specifying the Policy Type, of which there's a known set of values).
4. Having spent some time investigating how the API behaves however, it appears that the policy block doesn't accurately model the API behaviour - as such we'll need to make these top-level fields instead.
5. This should be using a Data Plane SDK from tombuildsstuff/kermit (or hashicorp/go-azure-sdk for Resource Manager endpoints) - at this point importing Track1 SDK's is adding technical debt - importing this into either kermit or go-azure-sdk means this doesn't need to be updated later.

This PR addresses most of these issues, with the exception of number 5, which is available via tombuildsstuff/kermit (as of v0.20230424.1090808) - but isn't part of this PR.

Fixes #21508

[tombuildsstuff]

Tests pass:

[manicminer]

LGTM 📘

[github-actions]

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[github-actions]

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