Simple Payments Block: Fix availability

[ockham]

Untested Counterpart to @enejb's D21761-code, to get files back in sync. Copying that diff's description:

Previously when we called the load blocks methods the call is being done too early in the rest api context. Which resulted in the api returnig the wrong data because we haven't switched to the correct blog contexts. We were always returning the availability as if we from the public-api.wordpress.com domain.

Changes proposed in this Pull Request:

This PR fixes this by making sure we load/register the blocks in the right context. And by allowing the blocks to pass a callback that gets called once in the right context ensuring that the data is as expected.

Testing instructions:

• Start with a Jetpack test site with a free plan
• Install either the REST API Console plugin or the Basic Authentication plugin.
• Use either plugin to run a GET request to /wp-json/wpcom/v2/gutenberg/available-extensions
• the simple payments block should be unavailable
• Now buy a Jetpack Premium plan
• Run the same GET request again
• the simple payments block should be available

[matticbot]

D21832-code. (newly created revision)

[jetpackbot]

	Warnings
⚠️	"Proposed changelog entry" is missing for this PR. Please include any meaningful changes

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[dereksmart]

@ockham this changeset was already deployed to wpcom, why still labeled in progress here?

[ockham]

@ockham this changeset was already deployed to wpcom, why still labeled in progress here?

Uh, good point. Mostly to signal that I hadn't tested this (and that it might require fixing to make it actually work). Should I set it to 'Needs Review' regardless?

(Not sure I'll get to it by EOW, I mostly filed it so it wouldn't fall through the cracks and so I could come back to it later.)

[dereksmart]

Worked like a charm!