Middleware restructure

[adam-fowler]

• Rewrite middleware to pass request onto next middleware instead of just editing request/response and returning it. eg

  struct MyMiddleware: AWSMiddlewareProtocol {
  public func handle(
      _ request: AWSHTTPRequest, 
      context: AWSMiddlewareContext, 
      next: (AWSHTTPRequest, AWSMiddlewareContext) async throws -> AWSHTTPResponse
  ) async throws -> AWSHTTPResponse {
      let request = doStuffToRequest(request)
      let response = try await next(request, context)
      return doStuffToResponse(response)
  }
  }

• Use result builder AWSMiddlewareStack to build middleware stack
• Convert each stage of the Soto execute chain to a middleware eg signing, retry, error handling, endpoint discovery
• execute is now
  • create request,
  • run middleware on request, passing final request to http client
  • process response returned from middleware to create output object

[codecov[bot]]

Codecov Report

Merging #570 (74a4123) into 7.x.x (74a4123) will not change coverage. The diff coverage is n/a.

:exclamation: Current head 74a4123 differs from pull request most recent head be1c747. Consider uploading reports for the commit be1c747 to get more accurate results

@@           Coverage Diff           @@
##            7.x.x     #570   +/-   ##
=======================================
  Coverage   78.86%   78.86%           
=======================================
  Files          73       73           
  Lines        6467     6467           
=======================================
  Hits         5100     5100           
  Misses       1367     1367           

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