[FEAT] Integrate state capture before host function calls

[Zhaenyra]

Description

This PR implements a state capture mechanism that records the ledger state immediately before the Soroban VM executes a host function, addressing Issue #979.

Changes

• Created simulator/src/host.rs with take_snapshot and a hook mechanism to record ledger state and instruction index.
• Updated simulator/src/vm.rs with prepare_host to register the hooks.
• Updated simulator/src/main.rs to declare the host module and initialize it.
• Updated simulator/src/runner.rs with a SimHost::take_snapshot() method.
• Updated simulator/src/lib.rs to export the new module.

Verification

• Verified snapshotting logic captures ledger entries, timestamp, and estimated instruction index.
• Logged snapshot IDs for debugging.

Fixes #979

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