Blocks: add block enqueuing methods

[jeherve]

Changes proposed in this Pull Request:

• Add methods allowing to enqueue assets for Jetpack blocks and standalone blocks.
• This PR does not switch existing blocks to use the new method; this will be done in a follow-up PR.

Jetpack product discussion

• See Primary issue: Block Infrastructure: allow building blocks separately #17099

Does this pull request change what data or activity we track or use?

• No

Testing instructions:

• Do the tests pass?

Proposed changelog entry for your changes:

• N/A

[jetpackbot]

Scheduled Jetpack release: November 10, 2020.
Scheduled code freeze: November 3, 2020

E2E results is available here (for debugging purposes): https://jetpack-e2e-dashboard.herokuapp.com/pr-17262

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Generated by 🚫 dangerJS against 715638f

[jeherve]

Closing this for now, since I haven't been able to work on it in the past few months.