allow headers and params input to generated code

[iscai-msft]

This issue is blocked by the release of azure-core (will be released 3/3/2022), because we want to use case_insensitive_dict, which was added in the latest release.

We want to allow users to pass in headers and params as kwargs to generated operations. Additionally, we want individually specified parameters to override these kwargs. For example, if we have foo query param to operation filter, if a user passes in filter(foo="bar", params={"foo": "should not be me"}, we should end up using the value bar because it was individually specified.

We want all of this to be case insensitive, which is why we need case_insensitive_dict from azure-core. The code for this is as follows:

1. I have an old-ish PR where you can find pretty much all of the code. The difference is this old code did not rely on case_insensitive_dict and used vendored code _get_from_dict. Please remove all vendored code and rely fully on case_insensitive_dict, otherwise the code should pretty much work!
2. I added additional tests to make sure the behavior of these kwargs is correct, see here. I've added sync and async for each generator, so please be sure to have these tests as part of the PR