documentloaders: add AssemblyAI document loader

[marcusolsson]

This PR adds the AssemblyAI document loader from LangChain and LangChain JS.

Unfortunately, due to the number of supported formats (in formatTranscript()), I had to increase the cyclop max complexity from 10 to 12. I first tried to break up the switch case, but then the exhaustive check failed.

I could also split the loader into a loader for each format. Let me know if you'd prefer it that way.

PR Checklist

• Read the Contributing documentation.
• Read the Code of conduct documentation.
• Name your Pull Request title clearly, concisely, and prefixed with the name of the primarily affected package you changed according to Good commit messages (such as memory: add interfaces for X, Y or util: add whizzbang helpers).
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. Fixes #123).
• Describes the source of new concepts.
• References existing implementations as appropriate.
• Contains test coverage for new functions.
• Passes all golangci-lint checks.

[marcusolsson]

FWIW I don't think the GitHub workflow use the .golangci-lint.yaml configuration. When I run golangci-lint run -c .golangci.yaml locally, I'm getting ~13 linter errors across the codebase.

[tmc]

Looking great, can you attach comments on all new exported symbols?

[tmc]

Not huge on bumping the max limit here but it seems ok, I filed #678 to address dev/CI linting config.

[marcusolsson]

Thanks! 🙏 Is there anything else you'd like me to do before this can be merged?

[tmc]

Nope, going to merge!