Support API and patch versions in charmhub fetch_library and fetch_libraries

[Alex Lowe]

More details: https://docs.google.com/document/d/1W6vpXDgom9HLGWNblnnwJIAcM9Lx_liJ56mIJ-ONujc/edit

The `fetch-library` charmhub endpoint should accept an api and optional patch version, like so:

```
/v1/<namespace>/libraries/<name>/<library_id>?api=<integer>&patch=<integer>
```

If patch is provided, api is necessary; however, if api is provided without providing patch, it will retrieve the latest revision that matches that api version.

Likewise for `fetch_libraries` it should be able to accept JSON like this:

```
[
  {
 "charm-name": "some-charm",
 "library-name": "latest-lib",
  },
  {
 "charm-name": "some-charm",
 "library-name": "api-specified",
 "api": 1,
  },
  {
 "charm-name": "some-charm",
 "library-name": "api-and-patch-specified",
 "api": 1,
 "patch": 0
  }
]

```

with the same requirements as `fetch_library`