[AMQP] Add transaction coordinator and transactional state support

[Raushan Singh (raushan728)]

Fixes #4937

Adds transaction support to azure_core_amqp, behind a transaction feature flag. Tier 0 for #4934.

A few decisions worth flagging for review:

• Used OwnedTransaction (attaches its own control link per transaction) instead of holding a persistent Controller — the latter would need a borrowed Transaction<'t> stored in a map, which isn't safe without an unsafe cast. Side benefit: a transaction spanning a reconnect fails cleanly on its own, which the issue asked for.

• detach() explicitly discharges any remaining transactions instead of just dropping them — OwnedTransaction doesn't send a real AMQP frame on drop, so relying on that would mean rollback only happens after a broker timeout.

• Transaction state is shared per-session (Arc<Mutex<...>> on the session), not global, so separate sessions don't see each other's transactions.

• CoordinatorIsNotImplemented now maps to its own error kind instead of the generic transport error bucket, so callers can distinguish "broker doesn't support transactions" from other attach failures.

Open question: should this target main, or is there a feature branch for the Service Bus GA epic? Let me know and I'll retarget.

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Thank you for your contribution Raushan Singh (@raushan728)! We will review the pull request and get back to you soon.