feat: router

[alanshaw]

This PR adds a router interface:

/**
 * The routing service is responsible for selecting storage nodes to allocate
 * blobs with.
 */
export interface RoutingService {
  /**
   * Selects a candidate for blob allocation from the current list of available
   * storage nodes.
   */
  selectStorageProvider(
    digest: MultihashDigest,
    size: number
  ): Promise<Result<Principal, CandidateUnavailable | Failure>>
  /**
   * Returns information required to make an invocation to the requested storage
   * node.
   */
  configureInvocation<C extends BlobAllocate | BlobAccept>(
    provider: Principal,
    capability: C,
    options?: Omit<UCANOptions, 'audience'>
  ): Promise<Result<Configuration<C>, ProofUnavailable | Failure>>
}

Tests implement a storage provider node and setup 2 nodes that are used in tests that the routing service picks randomly from. However the router will be sticky when asked to store the same blob multiple times. This allows existing tests for allocation and accept responses to pass. Note: we need to review if the semantics for allocate and accept responses really makes sense in a context where there are multiple storage providers (TLDR; the allocation size does not).

[alanshaw]

I'm merging this as I need to start work on upload-service-infra done before Devcon. However please feel free to leave review and I will address the feedback. Apologies this is a monster PR that touches many files.