internal: Add base HTTP client

Created by: efritz

Like the design of the dbutil basestore, this base HTTP client forms an abstract set of common properties that we can use for service-to-service communication. This was abstracted from the bundle manager client, and the client that we use to interact with the frontend queue API from within the precise-code-intel-indexer-vm.

I feel that these kind of base layers are useful for layering other cross-cutting concerns on top (metrics, tracing, logging, circuit breaking, rate limiting, service-level routing, generic authentication, etc).

This is a pre-step to the effort tracked in https://github.com/sourcegraph/sourcegraph/issues/14822.