feat: opa plugins, add the ability to set headers

[chusov]

Description

opa plugin can set headers if the policy response is false - https://apisix.apache.org/docs/apisix/plugins/opa/#using-custom-response
I would like to be able to set headers for a request if the policy response is true.
Use cases - to send to the backend the status of the policy, the policy name, fields from the JWT token.
The changes are not so big, so I can prepare a MR

[Revolyssup]

@lingsamuel Please assign it to me.

[chusov]

@Revolyssup
opa can also return:

response_headers_to_add (optional): an object mapping a string header name to a string header value
request_headers_to_remove (optional): is an array of string header names

https://www.openpolicyagent.org/docs/latest/envoy-primer/#output-document

[Revolyssup]

@Revolyssup opa can also return:

response_headers_to_add (optional): an object mapping a string header name to a string header value request_headers_to_remove (optional): is an array of string header names

https://www.openpolicyagent.org/docs/latest/envoy-primer/#output-document

Is this part of your use case?

[chusov]

Is this part of your use case?

not really) sorry

[Sn0rt]

@Revolyssup pls help me to inspect how the OPA server works. and which HTTP header is must pass to the OPA server

[Revolyssup]

@chusov Will you take a look at this PR to see if it fits your use case? #9710