Add mock-relay/builder to collection

[parithosh]

We usually want to test mev-boost codepaths in testnets and require a relay/builder infrastructure setup. This is mostly handled by external parties, but they join relatively late in the testing pipeline. In order to catch bugs earlier, we would need to spin up a mock-relay that would rely on a local node to act as a builder. This mock-relay can also modify the bid to make sure the relay provided block is used and it can additionally submit invalid blocks to cause some chaos on the network.

The mock-relay is a piece of software used in hive that we are reusing for our testing. The codebase can be found here: https://github.com/ethereum/hive/tree/master/simulators/eth2/common/builder/mock

The Dockerfile for the mock-relay can be found here: https://github.com/marioevz/hive/blob/builder-as-external-command/simulators/eth2/common/DockerFile.mock_builder

Few dependencies before this issue is addressed:

• Add ability to build the mock_builder as a standalone tool, it currently builds as a package on hive main and we used a fork in the past
• Add dockerfile for mock_builder into hive itself
• Consider adding some automation for building the tool and pushing to ethpandaops
• Decide sane defaults for the mock_builder

[parithosh]

So we no longer need to pull it from hive, its its own tool now: https://github.com/marioevz/mock-builder

just needs a dockerfile(old one that's linked above could be reused) and the remaining steps to add a role for it