Simplify event stream message signer configuration (#2671)

## Motivation and Context

This PR creates a `DeferredSigner` implementation that allows for the
event stream message signer to be wired up by the signing implementation
later in the request lifecycle rather than by adding an event stream
signer method to the config.

Refactoring this brings the middleware client implementation closer to
how the orchestrator implementation will work, which unblocks the work
required to make event streams work in the orchestrator.

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