Add ExpressionTemplateValueProvider

[mheath]

Implements a simple mechanism for providing custom expression template values from enums used in expression templates.

Closes gh-17447

[jzheaux]

Thanks, @mheath, for the PR! It's nice to hear from you again. I've left some feedback inline.

[mheath]

Hey Josh. Thanks for taking a look at my PR. It's been too long. We need to catch up sometime in more than just PR comments!

[jzheaux]

Thanks for the updates, @mheath. Are you able to sign your commit?

(Note that I rebased, squashed, and added a polish commit before I noticed. Apologies, you may need to reset in order to push a change to your commit message.)

[mheath]

Yep. My commit is signed.

[jzheaux]

This is what the CI sees:

Commit sha: 0b963d3, Author: Mike Heath, Committer: Mike Heath; The sign-off is missing.

In addition to the title and the reference to the ticket number, the commit message should say something like:

Signed-off-by: ...

I'm not sure why the commit in the PR doesn't have that. I'm sure you are already familiar, but just for easy reference, here is where GitHub reviews the DCO process.

[mheath]

Oops. I forgot that --gpg-sign doesn't add the DCO line. It's there now.

[jzheaux]

Excellent, @mheath. This will merge once the build completes. Thanks again!