feat: Add a canonical way to bring lamports into the validator

[GabrielePicco]

Add a canonical way to bring lamports into the validator

Status	Type	⚠️ Core Change	Issue
Ready	Feature	No	#41

Problem

See #41

Issues

Closes #41

Greptile Summary

This PR adds functionality for managing ephemeral lamports in the validator through PDAs, introducing a canonical way to handle temporary balances with top-up, delegation, and closure operations.

• Added new EPHEMERAL_BALANCE PDA seed in src/consts.rs for escrow lamports management
• Implemented three core operations in src/processor/top_up_and_delegate.rs: top-up (fund PDA), delegate (to program), and close (PDA)
• Modified buffer PDA validation in src/processor/delegate.rs to support uninitialized buffers for lamport-only delegations
• Added comprehensive test coverage in tests/test_top_up.rs for the full lifecycle of ephemeral balance operations
• Introduced new instruction types (TopUp, DelegateEphemeralBalance, CloseEphemeralBalance) in src/instruction.rs

[greptile-apps]

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