Improvement: deepgram "multi" language support and keyterms check

[ChenghaoMou]

1. nova-3/general and nova-2 supports a "multi" language where speakers can do code switch.
2. keyterms is only supported by nova-3, a warning or error should be raised since we downgrade nova-3 to nova-2 automatically with a check. Having keyterms in nova-2 requests will cause some issues in my testing.

Opening this so I don't forget to create PRs. Happy Friday!

[davidzhao]

yes! I ran into this yesterday and we have a pending PR for 1 against 0.x. If you could like to tackle 2, that'd be amazing!

[schedawg74]

Also nova-3 added multiple new languages besides english. Is this part of the PR as well?

[davidzhao]

@schedawg74 that'll be merged soon as well