Handle unknown values in deployment snapshot repository references

[tobio]

Description

Previously, the code was setting repository.reference: null in state for ESS deployments. This means that UseStateForUnknown does not actually set the planned value, leaving it as unknown. When TF comes back to parse this into the ElasticsearchSnapshotRepositoryReference struct it can't and the whole thing falls apart.

This PR updates the code to only set the repository reference value if it exists. In reality this just moves the problem up one level to the repository field, but we introduce a TF type where the repository support unknowns via types.Object, updating the consuming code accordingly.

Related Issues

Fixes #665

Motivation and Context

How Has This Been Tested?

Manually, I don't understand why this wasn't failing the existing acceptance tests. I'm clearly missing something, as I would have expected any updates to hit this error (that's my manual repro).

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (improves code quality but has no user-facing effect)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation

Readiness Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed