Bug 20402: Implement OAuth2 authentication for REST API

Bug 20402: Implement OAuth2 authentication for REST API

It implements only the "client credentials" flow with no scopes
support. API clients are tied to an existing patron and have the same
permissions as the patron they are tied to.
API Clients are defined in $KOHA_CONF.

Test plan:
0. Install Net::OAuth2::AuthorizationServer 0.16
1. In $KOHA_CONF, add an <api_client> element under <config>:
     <api_client>
       <client_id>$CLIENT_ID</client_id>
       <client_secret>$CLIENT_SECRET</client_secret>
       <patron_id>X</patron_id> <!-- X is an existing borrowernumber -->
     </api_client>
2. Apply patch, run updatedatabase.pl and reload starman
3. Install Firefox extension RESTer [1]
4. In RESTer, go to "Authorization" tab and create a new OAuth2
   configuration:
   - OAuth flow: Client credentials
   - Access Token Request Method: POST
   - Access Token Request Endpoint: http://$KOHA_URL/api/v1/oauth/token
   - Access Token Request Client Authentication: Credentials in request
     body
   - Client ID: $CLIENT_ID
   - Client Secret: $CLIENT_SECRET
5. Click on the newly created configuration to generate a new token
   (which will be valid only for an hour)
6. In RESTer, set HTTP method to GET and url to
   http://$KOHA_URL/api/v1/patrons then click on SEND
   If patron X has permission 'borrowers', it should return 200 OK
   with the list of patrons
   Otherwise it should return 403 with the list of required permissions
   (Please test both cases)
7. Wait an hour (or run the following SQL query:
   UPDATE oauth_access_tokens SET expires = 0) and repeat step 6.
   You should have a 403 Forbidden status, and the token must have been
   removed from the database.
8. Create a bunch of tokens using RESTer, make some of them expires
   using the previous SQL query, and run the following command:
     misc/cronjobs/cleanup_database.pl --oauth-tokens
   Verify that expired tokens were removed, and that the others are
   still there
9. prove t/db_dependent/api/v1/oauth.t

[1] https://addons.mozilla.org/en-US/firefox/addon/rester/

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>