[FEAT] Add Guide on how to use HTTP Builtin from OPA

[Parsifal-M]

Is your feature request related to a problem? Please describe.
No

Describe the solution you'd like
Some documentation and examples on how to use http builtin to for example fetch a users entity information in order to make more detailed rules.

Describe alternatives you've considered
N/A

Additional context
Imagine you want to build a rule based on an annotation that a user entity entity has, currently the permissions framework does not send the full user data, only the users name and claims.

Using the HTTP builtin, we can make a get request to the backstage catalog and fetch the users entity information like the metadata including annotations etc... which will be really useful for some rules.

[Parsifal-M]

We'd still want this!