Add `.convert` functionality to `ManagerAccessMixin`

[fubuloubu]

ideas (outside PR): make this operation and alike more convenient in ConversionManager and ManagerAccessMixin, e.g.:

self.convert(safe_tx_kwargs.get("safeTxGas", 0), int)
self.convert.to_address(safe_tx_kwargs.get("refundReceiver"))  # Where None is also ZERO_ADDRESS

Originally posted by @antazoey in ApeWorX/ape-safe#90 (comment)

[linear]

APE-2018