add SAS authentication option for Azure repos

[rgammans]

What does this PR change? What problem does it solve?

This PR adds the ability to use Azure SAS tokens for authentication. This feature is requested in #2295

This change adds an additional string value to the azure configuration struct to store the SAS, and a separate path to create the service client.

Was the change previously discussed in an issue or on the forum?

Fixes: #2295

Checklist

• I have read the contribution guidelines.
• I have enabled maintainer edits.
• I have added tests for all code changes.
• I have added documentation for relevant changes (in the manual).
• [ x] There's a new file in changelog/unreleased/ that describes the changes for our users (see template).
• I have run gofmt on the code in all commits.
• All commit messages are formatted in the same style as the other commits in the repo.
• I'm done! This pull request is ready for review.

I haven't coded any tests as they will require an update to the testing infrastructure to create SAS TOKEN, (although that should be a relatively straightforward change in the workflow). Functionally they should do the same as the existing tests but with a SAS token provided instead of ACCOUNT_KEY.

NB: I'm a relative newbie in Go so there may be some style issues.

[MichaelEischer]

LGTM.

I've rebased the PR, squashed the commits, fixed the merge conflict, fixed a few typos and added a commit to automatically strip a leading ? from the SAS token.