soroban-rpc: libpreflight: Remove base64 encoding between Go and Rust

[2opremio]

Addresses most of stellar/stellar-rpc#49

It doesn't yet address this though, which is hard since, in order to check expirations, we sometimes need to unmarshallthe entry anyways. I will revisit this when once the new expiration changes land.

The ledger entry is obtained from the DB in binary-serialized XDR

Before:

goos: darwin
goarch: arm64
pkg: github.com/stellar/soroban-tools/cmd/soroban-rpc/internal/preflight
BenchmarkGetPreflight
BenchmarkGetPreflight/In-memory_storage
BenchmarkGetPreflight/In-memory_storage-12         	    4802	    239106 ns/op
BenchmarkGetPreflight/DB_storage
BenchmarkGetPreflight/DB_storage-12                	    3680	    304496 ns/op
BenchmarkGetPreflight/DB_storage,_restarting
BenchmarkGetPreflight/DB_storage,_restarting-12    	    3664	    304327 ns/op
BenchmarkGetPreflight/DB_storage,_no_cache
BenchmarkGetPreflight/DB_storage,_no_cache-12      	    2146	    527755 ns/op
PASS

After:

goos: darwin
goarch: arm64
pkg: github.com/stellar/soroban-tools/cmd/soroban-rpc/internal/preflight
BenchmarkGetPreflight
BenchmarkGetPreflight/In-memory_storage
BenchmarkGetPreflight/In-memory_storage-12         	    6972	    168184 ns/op
BenchmarkGetPreflight/DB_storage
BenchmarkGetPreflight/DB_storage-12                	    4916	    229217 ns/op
BenchmarkGetPreflight/DB_storage,_restarting
BenchmarkGetPreflight/DB_storage,_restarting-12    	    4800	    229573 ns/op
BenchmarkGetPreflight/DB_storage,_no_cache
BenchmarkGetPreflight/DB_storage,_no_cache-12      	    2594	    431494 ns/op
PASS

[leighmcculloch]

One comment on something that I initially misunderstood, but does not need addressing.

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