Add wallet classes

[astrojarred]

This is still a WIP but I wanted to get the ball rolling on this! Lots of basic functionality is now here, along with the first few tests and examples. See /examples/wallet.py for some examples of the most basic functionality.

Current capabilities:

• Generate keys
• Load keys
• Query utxos
• Send ada
• Send specific UTxOs
• Get senders of all UTxOs
• Get metadata for all held tokens
• Get utxo block times and sort utxos
• Mint / Burn tokens
• Automatically load in token polices where wallet is a signer
• Automatically create BlockFrost Chain Context (mainnet, preprod, and preview)
• Attach messages to transactions
• Sign messages
• Add custom metadata fields
• Multi-output transactions
• Register wallet
• Stake wallet
• Withdraw rewards
• Generate fully manual transactions that can do any / all of the above
• Integrate strict type checking
• Write tests
• Add more examples

Future additions (possibly later PRs):

• Draft and sign multi-sig transactions
• Interaction with native scripts
• Interaction with plutus scripts
• Load wallets with mnemonic phrase (integrating HDWallet)

Now currently figuring out how to write tests for the transaction methods, using test_transaction.py for inspiration.

[astrojarred]

Quick update-- I've finally managed to integrate mypy so that the checks do not fail anymore. In addition I've gotten coverage up to ~44%. Still some tests to write but I'm closing in on this.
🔜