feat: add config option to disable swizzling

[ioannisj]

💡 Motivation and Context

Closes #362

Docs PR: PostHog/posthog.com#13204

• Adds a config option to disable swizzling
• Refactor PostHogSessionManager from singleton to SDK instance member in line with what js lib does

💚 How did you test it?

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

[ioannisj]

@marandaneto I'd like your take and thoughts on the following:

PostHogSessionManager is using swizzling here to detect user activity in the app as part of session rotation/expire logic.

I’d like to respect the config option to disable swizzling altogether in our code. How would you handle session manager here?

Options i see:

1. remove dependency on sendEvent() entirely.
2. first SDK instance that runs setup() decides swizzling for the common session manager
3. session manager picks up swizzling config from PostHogSDK.shared singleton
4. refactor so that each SDK instance runs its own session manager (and therefore inherits its config)

wdyt?

[marandaneto]

LGTM.
ps: just make sure the RN session replay plugin compiles and works correctly after the session manager changes