soroban-rpc: Add getPreflight benchmark and test

[2opremio]

No description provided.

[2opremio]

In the current benchmark (which simulates a hello world contract with in-memory ledger entries) I get:

goos: darwin
goarch: arm64
pkg: github.com/stellar/soroban-tools/cmd/soroban-rpc/internal/preflight
BenchmarkGetPreflight
BenchmarkGetPreflight-12    	    7196	    164246 ns/op
PASS

That is, 0.164246 ms per simulation! which seems overly good but maybe possible since the DB is not accessed.

I should now write a test accessing the DB.

[2opremio]

I have added a benchmark using the DB and there is a considerable difference:

goos: darwin
goarch: arm64
pkg: github.com/stellar/soroban-tools/cmd/soroban-rpc/internal/preflight
BenchmarkGetPreflightInMemory
BenchmarkGetPreflightInMemory-12    	    5947	    217767 ns/op
BenchmarkGetPreflightInDB
BenchmarkGetPreflightInDB-12        	    2659	    504930 ns/op
PASS

This indicates we should probably be caching the config entries with a write-through cache.

[2opremio]

note that the benchmark indicates that, even without caching, the preflight took 0.5 ms vs 0.2 milliseconds. So maybe it's not worth the optimization after all

[2opremio]

@tsachiherman PTAL