Support reading mysql backend credentials from environment variables

[miles-w-3]

Description

Adds ability to read in mysql backend storage credentials from environment variables. Updates the documentation to support this, as well as correcting the plaintext_connection_allowed - it seems like plaintext_credentials_transmission has not been a part of the implementation for a while

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[VioletHynes]

Hi there! Thanks for the contribution. This looks good to me! It looks like you need to sign the CLA, and I'll also need an approval from the documentation team before I can merge this.

[VioletHynes]

Oh, I'll also need you to add a changelog. This should be a file in changelogs called 30136.txt with something like the following text:

release-note:improvement
storage/mysql: Added support for getting username and password from the environment variables `VAULT_MYSQL_USERNAME` and `VAULT_MYSQL_PASSWORD`.

Feel free to look at the other ones in that folder for reference.

[miles-w-3]

@VioletHynes thank you for the review, I have added the changelog file and rebased to get the latest from main.

[VioletHynes]

@miles-w-3 you'll need to use triple backticks at the top and bottom of the changelog, e.g. 28121.txt might be a good example of what I mean. Apologies, it's hard to show the backticks on GitHub since they get construed as a codeblock.

[miles-w-3]

@VioletHynes My mistake, I should have looked at the other examples more closely. Should be fixed now, thanks.

[miles-w-3]

@VioletHynes apologies for my lack of familiarity with the vault release process. I noticed that v1.19.1 came out a few days after this was merged and did not include this fix. Is there a standard timeline for when new features get included vs minor release bugfixes, or did I just miss the cutoff?

Thanks

[VioletHynes]

Hi there! We don't generally backport new features or improvements, unfortunately, for a myriad of reasons. This will be released in 1.20. I know that might seem quite far away, and I do apologize for that, but that's unfortunately our standard practice to keep stability in dot releases.

[miles-w-3]

That makes sense, thank you