[extension/opamp]: Move custom message related interfaces to separate module

[BinaryFissionGames]

          I think this is good for an initial pass, but we should probably break this out into its own module at some point. It feels weird to pull in the whole extension module for a few interfaces.

Originally posted by @evan-bradley in #32281 (comment)

[github-actions]

Pinging code owners:

• extension/opamp: @portertech @evan-bradley @tigrannajaryan

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