make auth_header optional to enable using _request_session.headers

[zymergy]

Before this PR

requests provides a convenient Session object that allows headers to be shared across all requests made with a given session. conjure-python initializes all services with a requests.Session() object, but also created a non-optional auth-header field for all generated methods that require header authentication. Even if the auth header is specified in the Session object, it will not be respected and must be provided for every method call. This PR modifies the generation code to ensure that headers are optional, and will override the Service._request_session.headers object only if specified

After this PR

==COMMIT_MSG==
Required headers are now optional when calling methods and can instead be provided in the requests.Session.headers dict. If headers are provided in the method, they will override the session headers.
==COMMIT_MSG==

Possible downsides?

The exception raised for missing required headers will change from a TypeError to the exception raised by requests

[palantirtech]

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Description

Required headers are now optional when calling methods and can instead be provided in the requests.Session dict. If headers are provided in the method, they will override the session headers.

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[zymergy]

I suspect it would be helpful to add documentation pointing out that any headers in the method parameters are REQUIRED headers, they just don't have to be specified in the method call itself