Add RelayByTimestamp request to the relayer

[srdtrk]

Summary

Right now, all relaying and batching is done via tx hashes in RelayByTxRequest, which requires the submitters to submit tx hashes from both chains. This api is non-intuitive, and requires the users to know the tx hashes of IBC events to be able to relay.

Problem Definition

RelayByTxRequest works very well when it comes to 3rd party front-end integrators/api users such as skip go. It is not ideal when it comes to usage from end users or generic test suites.

Use cases

• Relay by only defining from and to unix timestamps.
• Test suites like interchaintest
• Makes it easier to implement an auto-relayer by running in a loop

Proposal

// The request message
message RelayByTimestampRequest {
    // The source chain identifier
    string src_chain = 1;
    // The target chain identifier
    string dst_chain = 2;
    // Unix timestamp to start batching from
    uint64 from_timestamp = 3;
    // Unix timestamp to stop batching.
    uint64 to_timestamp = 4;
    // The identifier for the target client
    // Used for event filtering
    string target_client_id = 5;
}

// The response message
message RelayByTimestampResponse {
    // The multicall transaction to be submitted by caller
    bytes tx = 1;
    // The contract address to submit the transaction, if applicable
    string address = 2;
}

---

For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged/assigned
• Estimate provided