[keysever/lib/native] extend joinThread to handle auto join functionality

Summary:
This diff introduces two new changes to joinThread to support auto join functionality

1. We introduce an optional defaultSubscription option where we can define what we want the default subscription of the thread to be when we initially join.
2. We introduce a new check to see if the thread the user is trying to join is a root thread that has a farcaster channel tag which means it is "discoverable" and we can bypass the standard threadPermission check

Depends on D12444

Test Plan:
EDIT: Confirmed that I was still getting the same result as the video below. Also confirmed that a user can still join a thread through the standard join thread button

Please see the demo video below

Reviewers: ashoat, kamil

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12459