Add back method which was removed in #3969

[bdukes]

Summary

The public method Mail.ConvertToText was accidentally removed in #3969, this PR adds it back.

[mitchelsellers]

All for this because it shouldn’t have been removed.

But going forward. Should we commit to having this method?

[bdukes]

I think it's a helpful method (we discovered it was missing because @donker was using it). However, we are duplicating the functionality (#3969 moved this to be a private method of CoreMailProvider). It would be nice to adjust one of the call sites to use the other (which, in this case, would probably mean adjusting CoreMailProvider to call Mail.ConvertToText).

This isn't functionality I would plan to directly use, so I can't comment on whether it's still a fit solution, so perhaps there's an alternative we should promote instead.

[david-poindexter]

@bdukes shouldn't this be targeted to the 9.8.1 release branch?

[bdukes]

Not necessarily. We can decide that's what we want to do. However, this is not a regression introduced by 9.8.1 (it was introduced in 9.8.0), so it doesn't meet the standard criteria for hot-fix adding to an active RC.

[donker]

Thanks!