Add a dynamic resolver factory for reusable bootstraps

[aryansk]

Motivation:
ClientBootstrap currently stores one resolver instance, but DNS resolvers are generally single-use. Reusing a bootstrap with a single-use resolver makes it difficult to issue multiple hostname connections safely.

Modifications:

• Add NIODynamicResolver, which creates a fresh resolver for each hostname connection.
• Allow ClientBootstrap to use the dynamic resolver in both synchronous and async hostname-connect paths.
• Add a concurrent connection regression test proving that each connection receives its own resolver and A/AAAA query pair.

Result:
A reusable ClientBootstrap can safely create fresh resolver instances for concurrent hostname connections. The factory boundary avoids the shared mutable pairing problem identified in the earlier closed PR #2553.

Testing:

• swift test --filter NIODynamicResolverTest/testCreatesFreshResolverForConcurrentConnections
• swift test --parallel
• git diff --check

Closes #2434