Consolidate validator manager utilities

[cam-schultz]

Context and scope
Many of the utility functions in validator_manager.go perform identical actions for PoA, PoS ERC20, and PoS Native token validator managers. The same functionality is split into different functions so that the ABI and types from the proper package in abi-bindings can be referenced. This results in a lot of repeated code that should be cleaned up.

Discussion and alternatives
Converting the utilities to use generics, or implementing a common interface in abi-bindings seem like two strategies likely to succeed. One (potentially) complicating factor is that the three generated concrete types in abi-bindings share many of the same inner type definitions defined in the parent Solidity contract (e.g. erc20tokenstakingmanager.PoSValidatorManagerSettings and nativetokenstakingmanager.PoSValidatorManagerSettings are identical)

Open questions
Questions that are still being discussed.