Add a method of listing the keys on a store

[James Jesudason]

`snapcraft list-keys` shows the keys in the local keyring and whether they are registered for an account. However, some customers have asked for the ability to list all the registered keys. Perhaps having a `--remote` option that will show the information from the snap store.

[Sergio Schvezov (sergiusens)]

We can do this on snapcraft's side as long as we have storeapi. I added a task for that team to either provide the documentation or implementation.

[Celso Providelo (cprov)]

Sergio,

The current API [1] already lists all the "remote" keys:

{{{
$ surl -a prod https://dashboard.snapcraft.io/dev/api/account | jq '.["account-keys"]'
[
  {
    "name": "serial",
    "since": "2019-02-01T05:01:32Z",
    "public-key-sha3-384": "6FUqPhWVUn0ocOrnO5Ip0HbNOI35sSZyAQ-ojExEXBR7_d0PKGnWR8xDIVHncJcX"
  },
  {
    "name": "new-default",
    "since": "2019-05-23T02:25:22Z",
    "public-key-sha3-384": "6Tm_gBSMHoZNaJ2qOGtOrWeNEkaNQcx5_tH67Utr5_qTMM5rQ3RW2IDWC01aW5g1"
  },
  {
    "name": "default",
    "since": "2016-09-12T08:43:28-03:00",
    "public-key-sha3-384": "K80uHIsxgUZU_2t4Kgs2Iv9CeyINK9V8N71U4didCxmt2NHmE7udzzmP389OBmAK"
  }
]
}}}

[Tony Espy (awe)]

@Sergio

I just discovered that if you're logged in and use `list-keys` on a system where no keys have been created, the user sees the message:

"No keys have been created on this system. See 'snapcraft create-key --help' to create a key.
The following SHA3-384 key fingerprints have been registered but are not available on this system:"

..and then shows the SHA3-384 fingerprints of keys that have been registered to the account.

So it seems the code's already there to do it, however it needs something to trigger it when local keys exist in the snap keyring (e.g. --remote or something close).