Create new Plans package

[leogermani]

This PR creates a new package with functionality to fetch Plans data from WPCOM.

For now, it copies the classes from the Jetpack plugin. Usage in the Jetpack plugin will be replaced in subsequent PRs as this must be synced with changes in WPCOM as well.

We need this to move the work on the My Jetpack feature.

Changes proposed in this Pull Request:

• Create new Plans package

Jetpack product discussion

N/A

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

No

Testing instructions:

• Nothing for now, the package is not used
• Let's see if the tests pass

[github-actions]

Thank you for your PR!

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[mkm119]

Restoring this branch since we need this package as part of making a standalone Publicize plugin.

Discussion: #23488 (comment)
p1647598368441399/1647597121.990859-slack-CBG1CP4EN