Add generic error handling and retry for temporary network failures

[corverroos]

Problem to be solved

Infura's beacon API is flaky, it returns a lot of temporary errors like timeouts and "Cannot create attestation for future slot".

Charon doesn't handle these temporary at all, which results in duties not being performed.

Proposed solution

• Add a "async function executor" that supports retries.
• Implement it in a new package app/retry
• Create new async root context, but continue from parent span.
• Retry until a duty has elapsed.
• Wire the retryer as wrapper to networking components.

Out of Scope

We should wrap each "broadcast to peer" in parsigex and leadercast in a similar retryer.DoAsync.