[Python] Add option to select/detect content-type.

[tomplus]

It's follow up the PR #10686 for python-legacy. It adds an extra argument _content_type to api calls to force a body content-type. It also extends select_header_content_type to return application/json-patch+json and help to override it in more complex scenarios.

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Thank you.

[spacether]

@tomplus you have a CI error for a python test:
FAILED test/test_fake_api.py::TestFakeApi::test_test_endpoint_enums_length_one
Can you fix it?
Once you fix it I can merge it. Your PR looks good otherwise. Thanks!

[tomplus]

It looks like all test passed, there is one error reported by ci/bitrise which is probably unrelated to changes.

[spacether]

Thank you for your PR; this looks good!

[spacether]

Maven error is unrelated