Add ConsistencyLevel and Executor

[evan-gray]

This PR adds the integrator-relevant files for Custom Consistency Level and Executor.

[nonergodic]

I'd also get rid of the natspec and only highlight what's actually important, because this comment from the README

NatSpec is avoided because it favors a "box-checking" approach towards documentation rather than focusing on essentials and rationales

was taking aim at exactly this sort of documentation:

/// @notice Returns the configuration for a given emitter address.
/// @param emitterAddress The emitter address for which the config has been set.
/// @return The configuration.
function getConfiguration(address emitterAddress) external returns (bytes32);

[evan-gray]

@nonergodic thanks for the feedback, I tried to match the comments style more and just had the one issue with makeAdditionalBlocksConfig if you could take another look, thanks!