Add a tx auth command

[Ifropc]

There are some functionalities related to auths that we were considering adding to different commands, but IMO it could get a bit clunky to find those subcommands or flags, instead adding a new command sounds like a more clear user story.
Currently, I can see 2 primarily functionalities that we wanted to add, related to auths:

1. Signing auths (originally part of feat: add tx sign/send and Stellar trait #1406)
   While it makes sense to add a new flag (--auth-only) to sign, moving it to a new command makes more sense it we want tx sign to have the same flags as other transaction signing commands. Moreover, not all transactions may have auth entries inside (in fact, most will not).
   As such, we can add tx auth <xdr> --sign-with-* command that only sings auths.
2. --dry-run for signing auths (Add a --dry-run option to tx sign #1486) nicely follows in line with ^
3. auth show <xdr> would be a great subcommand that lists all auths in this transaction, and shows which one is signed, which are not signed

cc @janewang @willemneal @leighmcculloch

[github-actions]

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