ACP 99 reference implementation improvements

[cam-schultz]

Why this should be merged

Improves upon #700 by making the following changes to align the rest of the validator manager contracts with ACP-99:

• Removes IValidatorManager, as ACP99Manager captures the functionality that validator managers must implement.
• Rename initializeValidatorRegistration, initializeDelegatorRegistration, etc methods to initiate to reflect the naming convention used in ACP-99. These methods initiate a process; they do not initialize any data. Note that initializeValidatorSet is not renamed, since this method does perform initialization.
• Rename delegator management methods in PoSValidatorManager to conform to the ACP-99 naming conventions for validator management.
• Remove ValidatorRegistrationInput in favor of providing the fields as standalone arguments. This matches ACP-99, as well as reduces the contract size.

How this works

See above

How this was tested

CI

How is this documented

The same changes above apply to the README