mod_http_oauth2: Add service documentation URL to metadata This is aimed to those building integrations, so the modules site seems appropriate. Configurable so that a deployment can point to their own OAuth documentation.

This module blocks configured queries that change server state.

E.g. to make vCard storage read-only:

``` lua
readonly_stores = {
	vcard = { "vcard-temp", "vCard" };
}
```