op-node: l2 rpc refactor

[protolambda]

Description

• Refactors previous l2.Source into sources.L2Client, now sharing a lot of commons with sources.L1Client through sources.EthClient
• Remove l2.ReadOnlySource: this was duplicate legacy code from the first op-proposer iteration. We have sources.L2Client now with caching + deduplicated code with L1 client.
• Add sources.EngineClient: wraps the sources.L2Client with the 3 engine API methods we use. (note: upstream execution APIs state that the eth namespace is part of the engine API endpoint in addition to the engine namespace, so this works).
• Refactor sources/types.go to:
  • Be more strict about blocks retrieved from post-merge RPC sources. Execution payloads are a subset of the valid block responses we can get.
    • As part of the above: fix the genesis setup in e2e to set the PoW nonce value to 0
  • Define a json definition for the header type. Could improve this more, but the intention is to remove big.Int out of the decoding/encoding picture, make the type flat instead of a pointer structure, and have more control to implement stricter checks on the contents.
• Port over and optimize Payload retrieval methods to EthClient. (Hopefully we can use this instead of block retrieval for L1 at some point. Especially if we can read raw transactions with some future RPC improvements, instead of the slow transaction encoding/decoding roundtrips we are forced to have now).
• Remove duplciate/outdated mockL2Client: testutils has everything any test may need for RPC mocking.

See previous L1/eth RPC refactor PR for more context: #3288

Metadata

Fix ENG-2280

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[mergify]

This PR changes implementation code, but doesn't include a changeset. Did you forget to add one?

[tynes]

Generally looks good to me, left some comments

[mslipper]

My feedback is non blocking. OK to merge as-is.

[tuxcanfly]

OK.

[mergify]

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