Allow block styling options for RRM buttons

## Feature Description

Similar to [an issue that was previously closed](https://github.com/google/site-kit-wp/issues/10875), provide styling options for the RRM button blocks. This is for when users wish to enable a CTA via a button, which we already support.

We already did provide styling option for the Sign in with Google block, this is specifically for RRM as the current implementation can look very out of sync on some websites.

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## Acceptance criteria

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## Implementation Brief

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### Test Coverage

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## QA Brief

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## Changelog entry

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Review issue #10875 and the existing styling options for the Sign in with Google block to understand the expected RRM button behavior. Identify the RRM button block entry points and related tests from those implementations; done means RRM CTA buttons expose comparable styling controls and no longer look out of sync with surrounding sites.

Written by the indexing model from the issue text.