simulators/eth2/common: create client interfaces, update mock builder

[marioevz]

This PR introduces the interfaces Client and ManagedClient which are now used by the ExecutionClient, BeaconClient and ValidatorClient structures to communicate with the clients.

Previously, the structures directly used hivesim.Client, which required a client launched and managed by hive.

hivesim.Client is now abstracted through ManagedClient to allow instantiation of ExecutionClient, BeaconClient or ValidatorClient and use an external client.

With this change, modules like the mock builder can now be imported by other non-hive modules, and even compiled separately as standalone commands.

Changes Included

Testnet Clients

• Client/ManagedClient interfaces
• Execution and Beacon clients now have separate configuration structures, ExecutionClientConfig and BeaconClientConfig respectively
• Beacon client structure can now automatically fetch the beacon spec from the client itself
• Beacon client subnets are now possible: If a beacon client is started with a specific subnet, their ENR will only be passed to the clients of the same subnet (useful to test forks and re-orgs)
• Testnet creation now uses configuration structs

Withdrawals Test Suite

• Tests now use the mock builder functionality to invalidate payloads

Mock builder

• Support to invalidate payloads, used in testnets to verify the circuit breaker behavior
• Support to create invalid outdated (from a previous fork) payloads.
• Mock builder behavior can now be altered while running by using the REST API (see builder/mock/README.md)

[holgerd77]

Hi @marioevz, thanks for all these extensive restructuring/refactoring works and efforts, really appreciated by "the users"!! 🤗 🤩 ❤️

[marioevz]

We could first merge #741 and then I could update the ManagedClient interface in this PR to include pause/unpause feature, which would allow to construct beacon chain tests which pause the beacon and validator clients. But let me know what you think @fjl

[marioevz]

I've been running these changes locally for quite some time now, and they are required to introduce more tests into the branch, so I will be merging now.