private key : embed wallet yet to add external wallets

[0xNoushad]

Pull Request Description

Related Issue

Fixes #55

Changes Made

This PR adds the following changes:

• Introduced BaseWallet for embedded wallet functionality.
• Added WalletManager for centralized wallet handling.
• Updated SolanaAgentKit to use BaseWallet instead of Keypair.

Implementation Details

• BaseWallet supports transaction signing with both secret keys and Keypairs.
• WalletManager ensures a single wallet instance and provides Anchor-compatible wallets.

Transaction Executed by Agent

Example transaction:

N/A

## Prompt Used  

N/A

## Additional Notes  
- Embedded wallet links to user identity for scalability.  
  
## Checklist  
- [x] I have tested these changes locally.  
- [ ] I have updated the documentation.  
- [ ] I have added a transaction link.  
- [ ] I have added the prompt used to test it.  

[slimeonmyhead]

Thanks for this change, I was able to use this to get a working example using Privy walletAPI (delegated actions and server wallets).

Many tools rely on a Keypair passed into the agent kit which makes this very annoying to fully integrate this change at the moment. I think this comment makes the most sense.