Add `REST_Conversion_Tracking_Controller` Class about site-kit-wp

[10upsimon]

AC ✅ Moving to IB

[eugene-manuilov]

IB ✔️

[mohitwp]

QA Update ✅

• Tested on dev environment.
• Verified when conversionInfra feature flag is disabled.
• Verified when conversionInfra feature flag is enabled.
• Verified calls response are as per QAB.

When 'ConversionInfra' feature flag is not enabled.

await googlesitekit.api.get( 'core', 'site', 'conversion-tracking' );
The result should be the default false enabled state: {enabled: false}

await googlesitekit.api.set( 'core', 'site', 'conversion-tracking', { settings: { enabled: true } } );
This should update the enabled setting to be true

Get the value again using await googlesitekit.api.get( 'core', 'site', 'conversion-tracking' );
It should be {enabled: true}

Validate that a non boolean value can not be set: await googlesitekit.api.set( 'core', 'site', 'conversion-tracking', { settings: { enabled: 'foo' } } );
Should result in an error with a 400 bad request status

Set the boolean value using integers: await googlesitekit.api.set( 'core', 'site', 'conversion-tracking', { settings: { enabled: 0 } } );

Call the value again using await googlesitekit.api.get( 'core', 'site', 'conversion-tracking' );
It should again be false as 0 is the boolean equivalent: {enabled: false}

Perform the same using 1: await googlesitekit.api.set( 'core', 'site', 'conversion-tracking', { settings: { enabled: 1 } } );

Call the value again using await googlesitekit.api.get( 'core', 'site', 'conversion-tracking' );
It should again be true as 1 is the boolean equivalent: {enabled: true}

[aaemnnosttv]

@10upsimon @tofumatt I left a comment on the PR with a question about the capabilities chosen here which seem off. See https://github.com/google/site-kit-wp/pull/8692/files#r1605582663

Closing this out as working, but let's follow up with an update if needed.