Make protobuf-src optional dependency of solana-storage-proto

[leoyvens]

Problem

The protobuf-src crate is inherently slow to run its build.rs as that compiles the C++ libprotobuf from source. As solana-storage-proto unconditionally depends on it, it is also slow to build.

solana-storage-proto in its build.rs already offers the ability to use system protoc by pointing to it in the PROTOC environment variable. However protobuf-src remains an unconditional depedency.

Proposed Solution

Make protobuf-src and optional dependency to solana-storage-proto.

[fanatid]

it's possible to use vendored crate: #8648