Use inline styles for Sign in with Google Gutenberg block

[tofumatt]

Summary

Addresses issue:

• Add Sign in with Google Gutenberg block to allow users to add Sign in with Google button to their site's layout #10046

Relevant technical choices

Uses inline styles as a stop-gap, because trying to load editor styles (or any styles) for this block when it's inside an iFrame using full-site editing does not work.

PR Author Checklist

• My code is tested and passes existing unit tests.
• My code has an appropriate set of unit tests which all pass.
• My code is backward-compatible with WordPress 5.2 and PHP 7.4.
• My code follows the WordPress coding standards.
• My code has proper inline documentation.
• I have added a QA Brief on the issue linked above.
• I have signed the Contributor License Agreement (see https://cla.developers.google.com/).

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Code Reviewer Checklist

• Run the code.
• Ensure the acceptance criteria are satisfied.
• Reassess the implementation with the IB.
• Ensure no unrelated changes are included.
• Ensure CI checks pass.
• Check Storybook where applicable.
• Ensure there is a QA Brief.
• Ensure there are no unexpected significant changes to file sizes.

Merge Reviewer Checklist

• Ensure the PR has the correct target branch.
• Double-check that the PR is okay to be merged.
• Ensure the corresponding issue has a ZenHub release assigned.
• Add a changelog message to the issue.

[github-actions]

Build files for 92086fd have been deleted.

[aaemnnosttv]

Thanks @tofumatt! This works great on the latest WP but I found issues (unrelated to this change) that we should address before rolling out this block to everyone. More detail below, but we should limit the block's availability to WP 5.8+.

[aaemnnosttv]

v2 was released in WP 5.6 with the addition of useBlockProps – I'm a little concerned that this may break things for folks on those older versions as this will be available to everyone. I'm not sure core has the concept of the API version at that time to prevent it from loading, but it actually errors when registered in JS right now due to a missing title.

Similarly, there is a notice raised on the backend due to the use of the block.json in the registration (I assume) as it's complaining about the lack of a prefix

Notice: WP_Block_Type_Registry::register was called incorrectly. Block type names must contain a namespace prefix. Example: my-plugin/my-custom-block-type Please see [Debugging in WordPress](https://wordpress.org/support/article/debugging-in-wordpress/) for more information. (This message was added in version 5.0.0.)

I'm not sure if this is blocking or not (no pun intended), but we should probably gate the existence of the block to WP versions 5.6+ to avoid causing problems, so long as we technically support older versions.

After some more testing, I found I wasn't able to use the block until WP 5.8! These are old versions, and we're overdue to raise our minimum requirement here, but we should avoid loading the block on versions that will only cause notices and ultimately be non-functional.

[aaemnnosttv]

I've pushed the relevant change to require WP 5.8+ for the block to be registered, so if all else passes then this should be good to go.