Update SiteCapabilities to use Transient class

[BrianHenryIE]

Proposed changes

The Transient class was created to address an issue where sites have an object cache present which breaks transients on the site. E.g. when a site has migrated in and continues to try to use a Redis instance that does not exist. It works by detecting object-cache.php and using wp_options table for transients if that is present.

The SiteCapabilities class was not using this implementation.

This PR updates SiteCapabilities to use the Transient class, and adds tests for both classes.

There is a small break where currently the wp_options based transients are stored with an expires key which has been renamed to expires_at. This means the transients will return false the next time they are fetched and then once set again will work as expected.

Type of Change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Video

Checklist

• I have read the CONTRIBUTING doc
• I have viewed my change in a web-browser
• Linting and tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

• There are still other classes which are using get_transient() etc. rather than the Transient class
• It may be possible to implement WP_Object_Cache as a global solution to the transient issue
• I tried defining set_transient() etc in the NewfoldLabs\WP namespace hoping calls from inside NewfoldLabs\WP\Module\Data\* would use that rather than \set_transient() but it only works for the same namespace and skips functions defined in intermediate namespaces

[wpscholar]

@BrianHenryIE - We should probably update the logic here. This is true for shared hosting where there is no object cache, but now that we have Bluehost Cloud that does have object caching, we should take this into account.

[BrianHenryIE]

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