Add [NonTransactional] attribute to opt out of auto-applied transactions

[jeremydmiller]

Summary

• Adds a new [NonTransactional] attribute that allows handlers and HTTP endpoints to explicitly opt out of transactional middleware when using AutoApplyTransactions()
• The attribute can be placed on individual handler methods or on the handler class itself
• Works for both message handlers and Wolverine HTTP endpoints

Closes #2180

Test plan

• MartenTests: [NonTransactional] on handler method prevents transactional middleware
• MartenTests: [NonTransactional] on handler class prevents transactional middleware
• MartenTests: Normal handlers still get transactional middleware with AutoApplyTransactions()
• Wolverine.Http.Tests: [NonTransactional] on HTTP endpoint prevents transactional middleware
• Wolverine.Http.Tests: Full test suite passes (474 passed, 10 skipped, 0 failed)

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