feat(flags): add proj feature observer

[michellewzhang]

followup to

• feat(flags): attach flag evaluations in sentry's frontend to events #77532
• ref(flags): prefix js flags #79445

add a feature observer for calls to project.features.includes(...) similar to the one we have for organization.features.includes(...).

this one behaves the same way as the old one. i refactored out the buffer updating code so we can reuse it for both cases. the flag evaluations for both org + project are pushed into the same flag queue, which is already being used in the beforeSend in initializeSdk.