offchain - rpc batch caller lib + use it for offramp rpc calls

[dimkouv]

Implemented the following interface

type EvmBatchCaller interface {
	BatchCall(ctx context.Context, blockNumber uint64, calls []EvmCall) ([]DataAndErr, error)
}

And created a new offRampReader method that utilizes it.

GetDestinationTokensFromSourceTokens(context.Context, []common.Address) ([]common.Address, error)

Limit of rpc batch calls is set to:

RPC_BATCH_LIMIT = 100

And with back-off policy four retries are going to be made each with the following batch sizes:

100, 20, 4, 1

---

	// back off multiplier defines the rate of reducing the batch size limit for retried calls.
	// For example if limit is 20 and multiplier is 4:
	// 1.        20
	// 2. 20/4 = 5
	// 3. 5/4  = 1

[mateusz-sekara]

Looks good to me. However, as a follow-up, I think it's worth investigating using the same value as the one provided in the toml file smartcontractkit/chainlink#9119

This way, maybe we could avoid backoff in the code because NOPs, in case of any issues, can lower the batch RPC limit themselves. @connorwstein wdyt? Worth asking core team about they opinion on this approach anyway.

[connorwstein]

Looks good to me. However, as a follow-up, I think it's worth investigating using the same value as the one provided in the toml file smartcontractkit/chainlink#9119

This way, maybe we could avoid backoff in the code because NOPs, in case of any issues, can lower the batch RPC limit themselves. @connorwstein wdyt? Worth asking core team about they opinion on this approach anyway.

Yeah I think at least standardizing on defaults per chain across all services makes sense. In that case I think the parameter would need to move out of Gas estimation to a more general place though. I think backoff code is straightforward enough prob worth to keep for its graceful degradation.