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

The base branch was changed.

[iansuvak]

Re-approving, changes since last approval are regenerated abi-bindings post merge.