Integrate TAP Agent with gateway registry contract

[aasseman]

Problem statement
Currently the TAP Agent relies on a user-provided list of sender addresses with corresponding TAP aggregator endpoint URLs. This is a stopgap solution for integration testing.

Expectation proposal
Once the gateway registry contract is implemented (TBD), TAP Agent will be modified to get the TAP aggregator endpoint URLs from there.

Alternative considerations
A clear and concise description of any alternative solutions or features you've considered.

Additional context

indexer-rs/tap-agent/src/agent.rs

Lines 70 to 73 in 1178c63

	// TODO: replace with a proper implementation once the gateway registry contract is ready
	let sender_aggregator_endpoints = aggregator_endpoints::load_aggregator_endpoints(
	config.tap.sender_aggregator_endpoints_file.clone(),
	);