Add a method of listing the keys on a store

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.

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"
  }
]
}}}

@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).