Get rid of private members from MiddlewareStack class

[AllanZhengYP]

Is your feature request related to a problem? Please describe.
Because of the limitation of TS, 2 classes with the same signature but with private member variables are not assignable. In V3 SDK the situation happens a lot. For example, when users import client-s3@1.0.0 that depends on middleware-stack@1.0.0, and s3-request-presigner@1.0.0 that depends on middleware-stack@2.0.0, they import difference version of dependency middleware-stack. Then NPM will install to difference MiddlewareStack class. Because they both contain private members, middleware instantiated from each class are not assignable.

This will be a problem whenever 2 middleware stack need to interact, like merging. So user will see TS errors when presign an S3 request with incompatible S3 client and S3 presigner version. See #1365

Describe the solution you'd like
Get rid of private variables/methods by:

1. make them public
2. convert the class implementation of MiddlewareStack to be pure functional.

[trivikr]

Fixed in #1398
Released in v1.0.0-gamma.6

[github-actions]

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