feat: refactor middleware stack

[AllanZhengYP]

Issue #, if available:
#1377

Description of changes:

• This change refactors the middleware stack class into a factory. So that the private data are stored in closure instead of class "private" members. Now, you can instantiate a middleware stack by constructStack<Input, Output>(); instead of new MiddlewareStack<Input, Output>().

• Remove step from the addRelativeTo options. This way the position of the middleware is solely depend on the toMiddleware and relation(before or after).

• Refactor the middleware stack resolution implementation. Now the resolution algorithm is easier to follow & maintain.

/cc @trivikr

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-commenter]

Codecov Report

Merging #1398 into master will increase coverage by 0.07%.
The diff coverage is 98.42%.

@@            Coverage Diff             @@
##           master    #1398      +/-   ##
==========================================
+ Coverage   79.08%   79.15%   +0.07%     
==========================================
  Files         287      287              
  Lines       11155    11092      -63     
  Branches     2360     2340      -20     
==========================================
- Hits         8822     8780      -42     
+ Misses       2333     2312      -21     

Impacted Files	Coverage Δ
packages/middleware-stack/src/MiddlewareStack.ts	98.38% <98.31%> (+10.68%)	⬆️
...re-bucket-endpoint/src/bucketEndpointMiddleware.ts	91.30% <100.00%> (ø)
...iddleware-sdk-s3-control/src/prepend-account-id.ts	86.95% <100.00%> (ø)
...dk-transcribe-streaming/src/middleware-endpoint.ts	95.23% <100.00%> (ø)
packages/middleware-signing/src/middleware.ts	91.66% <100.00%> (ø)
packages/smithy-client/src/client.ts	84.21% <100.00%> (ø)
packages/util-create-request/src/foo.fixture.ts	76.92% <100.00%> (ø)
packages/util-create-request/src/index.ts	100.00% <100.00%> (ø)

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