[feature/bugfix] Probe S3 storage for CSP uri, add config flag for extra URIs

[tsmethurst]

Description

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This pull request updates our CSP construction logic to add extra URIs from config, and s3 URI from probed storage. This should fix cases where the S3 endpoint differs from the domain where images etc actually end up getting served from, while also adding flexibility for instance admins to specify their own URIs if they wish.

Tested this locally by connecting the testrig to an actual S3 bucket, and it works OK :)

Closes #2123

Checklist

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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.

[daenney]

That's some good probing.