Implement content fetching

[AMDmi3]

For some urls (e.g. build logs) we want to fetch content. For that we need to store a flag in the database that the content is needed (either as another bool column or link type mask), pass the context with this flag through the processors and respect it in http processor by issuing a get request and storing the content in the database,

Additionally, we need to store etag in the database as well to avoid refetching the same content multiple times.