Python: Type-stub generation for SSDKs (#2149)

* Initial Python stub generation

* Handle default values correctly

* Only generate `__init__` for classes that have constructor signatures

* Preserve doc comments

* Make context class generic

* Put type hint into a string to fix runtime error

* Run `mypy` on CI

* Use `make` to build Python SSDKs while generating diffs

* Escape Python types in Rust comments

* Only mark class methods with

* Sort imports to minimize diffs

* Add type annotations for `PySocket`

* Dont extend classes from `object` as every class already implicitly extended from `object`

* Use `vars` instead of `inspect.getmembers` to skip inherited members of a class

* Fix linting issues

* Add some tests for stubgen and refactor it

* Add type annotations to `PyMiddlewareException`

* Fix tests on Python 3.7

Python 3.7 doesn't support reading signatures from `__text_signature__`
for non-builtin functions (i.e. C/Rust functions). For testing we're using
regular Python syntax for defining signature.

* Provide default values for `typing.Optional[T]` types in type-stubs

* Update `is_fn_like` to cover more cases

* Remove `tools/smithy-rs-tool-common/`

* Make `DECORATORS` an array instead of a list

* Ignore missing type stub errors for `aiohttp`