net/mail: parse group in email address

net/mail: parse group in email address

This change adds the ability to parse
group into email address. The information about
group name and group members is lost for
backwards compatibility. According to this rule address
`Group: Test <text@example.com>;` would be parsed into
`Test <test@example.com>`.

Fixes #22014

Change-Id: I6e804a62f3ede04f555a1b82500b8ca030eeb431
Reviewed-on: https://go-review.googlesource.com/66250
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>