feat: bulk proof generation

[solipsis]

As a larger protocol, we need to construct and claim thousands of proofs per week. It would be great if the API and libraries had a method for generating multiple proofs at once

  // existing library
  res, err := client.GenerateClaimProof(context.Background(), &rewardsV1.GenerateClaimProofRequest{
    EarnerAddress: earnerAddress,
    Tokens:        tokens,
  })

// possible new method accepting an earners array instead of a single value
  res, err := client.GenerateClaimProofs(context.Background(), &rewardsV1.GenerateClaimProofRequest{
    Earners:  earners,
    Tokens:        tokens,
  })