Headers policy: allow templating using liquid

[mikz]

Use Cases

Add a HTTP Header to the request that contains value parsed from the JWT used for authentication (#607 (comment)).

Using HMAC to hash and sign some value and pass it upstream in the HTTP Request Header.

Details

We should offer both "plain" and "liquid" replacement

It is possible some clients will have "{{ }}" inside their replacement and that would cause an issue.
But maybe that change is negligible and we should always template the string by "liquid".
And last concern is performance - templating it by liquid right now could have performance impact before we make it all JITable.

This can't be a breaking change to the policy configuration.

https://github.com/3scale/apicast/blob/5ec4848a8527143d3764d43bb0c447afdac5b52a/gateway/src/apicast/policy/headers/apicast-policy.json#L42-L45

We could let "value" to be a string or an object {"type":"plain|liquid", "value": "..."}, but that would not work with the UI library (https://github.com/3scale/apicast/issues/6950.

Another way would be to introduce another key "value_type" that is optional enum defaulting to "plain".

Other policies will try to do the same.

Try to keep similar interface and see how other policies need to use liquid templating. One other candidate is the rate limiting policy.