Add SRV DNS record support to HTTP Operator

[gtxu]

This pull request allows an HTTP connection resolve its target host and port from a DNS SRV record instead of a fixed host/port, so it can talk to services published via SRV-based service discovery(ie. AWS Service Discovery for ECS).

This changes introduces dnspython as an optional dependency to keep the core HTTP provider lightweight. To prevent DNS query flooding, a TTL cache mechanism is implemented.

• Implemented soft dependency via the srv extra.

• Built RFC 2782 compliant failover (lowest priority tier filtering and random target selection).

• Added concurrency-safe TTL caching (srv_cache_ttl) using asyncio.Lock to prevent cache stampedes in asynchronous contexts.

• Exposed srv_lookup and srv_cache_ttl configuration directly in the Airflow Connection UI via custom form widgets.

closes: #9186

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Was generative AI tooling used to co-author this PR?

• Yes - Claude Sonnet 5

Generated-by: Claude Sonnet 5 following the guidelines

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