api, cmd, prod, pss, swarm: get publisher and query feed for chunk repair request

[mortelli]

This PR is a WIP for the following task:

implement inheriting publisher info via the context.WithValue, construct and lookup the latest update of the pinners feed.

from issue #2161.

Implementation overview

• NewRecoveryHook constructor func now also receives a feeds handler as a param, so the actual recovery hook can use it for feeds.
  • the handler passed to this constructor is the one created in the NewSwarm function.
• prod.getPinners func now extracts a publisher string from the ctx param, gets the content from a feed using the address derived from the publisher, and unmarshals the content feed as targets to construct the trojan chunk.
• adds a publisher flag for swarm up to register the publisher in the file manifest
• adds a request header for upload with the publisher of the content

Tasks

• [x] add tests if possible
• [x] resolve TODOs or create new issues for them

[mortelli]

the manifests part of this issue is still not complete, but the first iteration for the feeds part is ready, so i am requesting preliminary reviews from @zelig and @santicomp2014.

[mortelli]

i am marking the testing task as complete due to the following reasons:

• the prod tests have been updated after making use of the feed functions
• the publisher extraction from the manifest is not worth testing due to the amount of layers/components involved. the manual testing @santicomp2014 is bravely doing + phase 3 will make it clear whether this is working or not

[mortelli]

how about extracting the non-controversial parts in one or two PRs which we just merge?

* Targets  type in trojan package

* feed testutils and handler interface

then rebase this PR and we figure out contexts and how publisher gets in there etc

added PRs #2178 and #2179 for this.

thank you for the very appropriate suggestion

[mortelli]

new related issue: #2180

[mortelli]

new related issue: #2181