API: Support key_id query param

In lookup/discover, add support for key_ids.

Currently I am doing a key fetch and then looking up users by key_fingerprint. Would be nice to skip the key fetch step. (BTW, this is for use case for looking up recipients of a message from key_ids used to encrypt, not signing or verifying, which I get the fingerprint from kbpgp a-ok.) Low priority, but nice to have.

```
https://keybase.io/_/api/1.0/user/lookup.json?key_id=0022045e363bcbc1,401ed93c8a2c0cf0
```

Start at the lookup/discover API endpoint and compare its current key_fingerprint handling with the key_id query example in the issue. Done means callers can pass one or more key IDs to look up recipients directly, without first fetching keys; the payload does not name implementation files or tests.

Written by the indexing model from the issue text.