Adding support for Gmail extensions

[CrispinStichart]

Gmail added a few non-standard extensions to the IMAP protocol, for handling labels and a few other things.

What's this project's stance on supporting non-standard extensions?

[jonhoo]

I'm not against adding these in, probably behind a trait of some kind so that users have to explicitly say they want access to those methods. Or maybe better yet, through a

impl Session {
    fn gmail(&mut self) -> Option<GmailClientExtensions<'_>> {}
}

which returns None if the appropriate capability is present on the server.

[CrispinStichart]

Okay, cool. I'll give it a shot. ^_^

[CrispinStichart]

Well, that was easy -- it turns out that support for Gmail labels (which was the only thing I care about) was added to imap-proto a couple of months ago.

Unfortunately, they haven't made a release yet that includes those changes.

In the meantime, I forked rust-imap, switched the imap-proto dependency to pull from the git, and did what needed to be done: CrispinStichart@6e5ec17

You'll notice I deleted NameAttributes entirely, because imap-proto implemented that with this commit: djc/tokio-imap@4f60707

[jonhoo]

@djc Any chance we could have a new imap-proto release with the label changes?

[djc]

I just published imap-proto 0.16 to crates.io.

[CrispinStichart]

Thank, djc!

I made a pull request for this: #225