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.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[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]

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