Add PY (Python Interface) foundation layer

Introduces foundational components for Python Interface (PY):
- Module structure (clive/__private/py/)
- Exceptions: PyContextManagerNotUsedError, validation errors
- Input validators: AccountName, PageSize, PageNumber, KeyPairs
- Data classes: Balances, Accounts, Authority, AuthorityInfo, Witness, WitnessesResult, KeyPair
- CommandBase for PY commands with validation support
- Foundation commands: ShowProfiles, ShowBalances, ShowAccounts, ShowWitnesses, ShowAuthority, GenerateRandomKey

The WitnessesResult dataclass provides proper encapsulation for witness queries,
returning witnesses list along with total_count and proxy metadata.