Stored response fetcher (with new param)

[VeronikaSolovei9]

Stored response fetcher

[VeronikaSolovei9]

Initial working implementation is ready for review.
Questions:

1. There are two types of data we fetch from DB now: "imp" and "request". Should I add new type "response" and all related functionality (update database.go -> sendEvents and fetcher.go -> FetchRequests functions)
2. I reused existing stored_requests.Fetcher for stored responses. Should I rename it or create new one?
3. There are no unit tests for this functionality. Do I miss something?

[hhhjort]

1. There are more than two types, there are also "video" and "category" items that can be fetched. The one thing they have in common is that they all fetch json blobs. Imp and request were initially intertwined, as when initially implemented there was no other type of fetcher contemplated. Since then, we have been trying to detangle them as we add more fetchers, and have a common codebase that all fetchers can use. So reuse existing code when it makes sense, and add response specific code as needed.
2. In router.go, I see a separate storedRespFetcher object, which is the point where we need a separate object. As I said above, if we can reuse fetcher code, we should do so. We might consider renaming the module from stored_requests to stored_objects, since the fetcher framework has already expanded beyond requests. But that is probably out of scope for this PR.
3. The purpose of this code is to provide the ability to do end to end tests without external dependencies. So you should be able to set up a test with an auction object configured with a simple file based stored response fetcher, feed it a request that invokes the stored response, and verify that it returns the expected bids.

[hhhjort]

Why are we using an empty fetcher? We should be able to use stored responses on the AMP endpoint I would imagine.

[VeronikaSolovei9]

I'm not sure about this yet. Do we need to support stored response functionality in amp endpoint?

[hhhjort]

Where is the code that checks for the stored response signal, and then bypasses sending a request to the bidder and just providing the stored response to the flow?

[VeronikaSolovei9]

This PR is only about stored response fetcher.
We decided to have smaller PRs that are easy to review rather than having big and complicated PRs.
I understand this fetcher is not connected to anything and has no usages.
I'll add actual code related to stored responses in another PR.

[mansinahar]

Closing this in favor of #2099. Will reopen if needed.