view: extend the ViewService with a method to query latest activity

[erwanor]

This issue takes place in https://github.com/penumbra-zone/penumbra

Docs: https://buf.build/penumbra-zone/penumbra/docs/main:penumbra.view.v1

Suggested API:

service ViewService {
  // …
  rpc LatestSwaps(LatestSwapsRequest) returns (stream LatestSwapsResponse);
}

message LatestSwapsRequest {
  // If present, filter balances to only include the account specified by the `AddressIndex`.
  core.keys.v1.AddressIndex account_filter = 1;
  // If present, filter balances to only include trading activity on the specified pair.
  core.dex.v1.DirectedTradingPair pair = 2;
  // If present, limit the responses to activity that occured after this block height.
  uint64 after_height = 3;
  // Limit the response to the last entries within `response_limit`.
  uint64 response_limit = 4;
}

message LatestSwapsResponse {
  core.dex.v1.DirectedTradingPair pair = 1;
  core.asset.v1.Value input = 2;
  core.asset.v1.Value output = 3;
  uint64 block_height = 4;
  core.txhash.v1.TransactionId id = 5;
}