Verify making a post in Python configuring the headers

[andreaTP]

Opening this as PR as it adds to coverage, but the intention is to focus on the HeadersCollection implementation.

[andreaTP]

This is the nasty detail, it looks like a lot of boilerplate for something that we used to write as:

config = TodosRequestBuilder.TodosRequestBuilderPostRequestConfiguration(
        headers = { "My-Extra-Header", "hello" }
 )

Either we add more helper methods to make the conversion automatic for the user or, at the very least, we should add a constructor from Dict[str, str | List[str]] to HeadersCollection.

What do you think?

[baywet]

for context to others reading this. It's being discussed here microsoft/kiota-python#208