feat: implement set header

[df-wg]

Motivation and Context

This PR expands our header propagation abilities, by allowing users to define a value to set on all request/response headers in the format:

headers:
  all: # Header rules for all subgraphs
    request:
      - op: "set"
        name: "X-API-Key"
        value: ${SECRET_FOR_SUBGRAPH}

The set operation works both for requests and responses, and it can be applied globally (to all requests on the router), or only on a specific subgraph.

TODO

• Tests or benchmark included
• Documentation is changed or added on https://app.gitbook.com/
• PR title must follow conventional-commit-standard

[jensneuse]

one minor comment