feat(transport): expose server implementation on transport

[NathanBaulch]

Description (what this PR does / why we need it):

Expose the server implementation on Transporter, similar to what's available in GRPC interceptors. This gives middlewares the ability to make decisions or delegate functionality to the server/handlers that do the work.

Example middleware that delegates authentication checks to the server handler:

func Server() middleware.Middleware {
	return func(handler middleware.Handler) middleware.Handler {
		return func(ctx context.Context, req any) (any, error) {
			if tr, ok := transport.FromServerContext(ctx); !ok {
				return nil, errors.Unauthorized()
			} else if auth, ok := tr.Server().(Authenticator); !ok {
				return nil, errors.Unauthorized()
			} else if ctx, err := auth.Authenticate(ctx, tr); err != nil {
				return nil, err
			} else {
				return handler(ctx, req)
			}
		}
	}
}

This is an additive change that requires HTTP servers to be regenerated since they need to call http.SetServer alongside the existing http.SetOperation.

[shenqidebaozi]

This change looks strange, I don't know why it's done this way

[NathanBaulch]

I'm more than happy to answer any questions. My approach exactly mirrors the way Transport.Operation (aka grpc FullMethod) is intercepted and exposed to middlewares.

[NathanBaulch]

For now I'm forced to work around this by post-processing the generated code with:

# hack: ensure http handlers capture server
sed -i 's/http.SetOperation/http.SetServer(ctx, srv)\n\t\t&/g' ../internal/api/server/v1/*_http.pb.go

[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.

Project coverage is 81.74%. Comparing base (4e9dac0) to head (3cca60d).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
transport/http/transport.go	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3279      +/-   ##
==========================================
- Coverage   84.12%   81.74%   -2.38%     
==========================================
  Files          89       92       +3     
  Lines        4182     4327     +145     
==========================================
+ Hits         3518     3537      +19     
- Misses        471      600     +129     
+ Partials      193      190       -3     

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