Explicitly pass OpenMlsProvider to metadata functions

[insipx]

This will allow caller to decide whether to instantiate new connection or pass existing one. reduces the number of connections used for list_members.

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Note all of the metadata methods for a group pull their own connection. These methods are generally for one specific thing, and are the sorts of methods that may generally be composed with alot of the same call (iterate over $X groups and get the group name for instance). if parallelized, these calls could possibly block the thread, since our SQLite pool is blocking. To mitigate this, we could 'reserve' a single SQLite connection, instantiate a XMTPOpenMlsProvider with it, and re-use it for all read-only metadata methods. This will reduce the amount of concurrent connections being pulled from the pool.