soroban-rpc: Implement ledger storage

[tsachiherman]

Per the output of the investigation done in #4731, implement the data store for ledger entries in soroban-rpc.

The desired outcome of this task is the data layer, which is capable of doing the following:

1. when a new ledger is closed and received via txmeta from the captive-core data stream, the data tier would write, atomically, the ledger key changes to a local storage.
2. The data tier would have the functionality of making non-blocking reads from the data storage, given a ledger key.
3. the data tier needs to have an atomic bootstrapping functionality, allowing it to bootstap all it's content from the captive-core.

Add ledger entry storage for soroban-rpc. This should include the work to wire this into simulateTransaction.

[mollykarcher]

Spike portion of this original ticket moved to #4731

[2opremio]

The implementation is done at stellar/stellar-cli#322