[feature] support Sec-Websocket-Protocol in streaming API

[matthewp]

Description

Implements Sec-Websocket-Protocol header for streaming.

closes #1248

Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: [ ] -> [x]

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[NyaaaWhatsUpDoc]

Have only taken a brief look but this looks really great :) I shall leave it up to @tsmethurst to perform a full review as I am quite limited in my time outside of work 😅

[tsmethurst]

Looks good, thank you! Couple of comments about a few small things related to tests + comments, but that's it :)

[matthewp]

@tsmethurst Updated based on your review

[tsmethurst]

great stuff, thank you!