Interpolate headers send by HTTP Action

[marcinus]

Is your feature request related to a problem? Please describe.
HTTP Action supports path interpolation and is soon to support body interpolation (string or JSON).
In principle, it is a possible use case that some header value (or maybe even header name) could be interpolated from ClientRequest/Fragments data, e.g.:

Service A responds with authorization token that needs to be passed in header `Auth-Token` when calling Service B.

Describe the solution you'd like
I'd like to have a possibility to enable header interpolation (interpolateHeaders flag?)
Best case, I'd like to create whitelist/blacklist for the headers that could be interpolated.
This should also cover the additional headers that I specify in the configuration.

Describe alternatives you've considered
It's feasible not to have header interpolation at all, but having this option allows to directly set header values based on:

• action's results
• endpoint responses
• HTTP proxy values
• static values
• combinations of the above

Without this functionality, it's not clear to me whether it can be overriden without writing a custom HTTP action, which would be painful for the developer using Knot.x.

Additional context
See #73 and related PR #120