My Jetpack: Add filter for avoiding initalization

[oskosk]

Introduces a method should_initialize on the initializer with jetpack_my_jetpack_should_initialize filter.

Changes proposed in this Pull Request:

• Introduces a method should_initialize on the initializer..
• Adds a WordPress filter jetpack_my_jetpack_should_initialize for avoiding initalization

For Rationale, see the request for comments on this P2 p1HpG7-eqp-p2

Jetpack product discussion

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

No

Testing instructions:

• On a new site, or old one and disconnect Jetpack
• Verify that the My Jetpack menu is not present
• Connect site at a site level only (Via the Jetpack plugin). Disconnect
• Verify that the My Jetpack menu appears
• Connect a user
• Verify that the My Jetpack menu appears
• Unset the constant JETPACK_ENABLE_MY_JETPACK and confirm the menu disappears. Set it back.
• Add some snippet somewhere with add_filter( 'jetpack_my_jetpack_should_initialize', '__return_false') and confirm My Jetpack does not load.

[github-actions]

Thank you for your PR!

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Feedback addressed

[retrofox]

LGTM