Add headers for multiple language identification

[mbatchkarov]

Issue #, if available: N/A

Description of changes:
Hi, Transcribe SDE here. We recently launched a new feature called multiple language identification. We've been asked to contribute to this package to enable the feature so customers can use it.

Notes
language_code is currently a required positional parameter. When language ID is added, language code should become optional. That means we'd have to make it the third param and give it a default value, but this would break existing clients that use positional-only arguments. Therefore I'm leaving it as a positional arg and requiring that it's set to None when language ID is enabled. I'm not too happy with this option either- happy to discuss. Maybe something like this would be more ergonomic:

        if identify_language or identify_language:
            warnings.warm("Setting language_code to None because language ID is enabled")
            language_code = None

Testing
I extended and ran the integration tests locally from my own AWS account.

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[mbatchkarov]

Update: just saw #89 - looks like plain language ID is also not supported. Will update the PR to include that too

[praveenXira]

When will this PR get merged? I really need multiple language identification feature.

[GameSetAndMatch]

I am not familiar with AWS coding standards, but is there a reason why there isn't type hiting on identify_multiple_languages and language_options ?

[GameSetAndMatch]

Definitely a feature I would use in the short term if it was reviewed and merged by AWS team, good work @mbatchkarov !