Rust generator may need to override JSON name

[coryan]

Rarely, protos have annotations to override the JSON encoding:

https://github.com/googleapis/googleapis/blob/bb6b53e326ce2db403d18be7158c265e07948920/grafeas/v1/sbom.proto#L57

Sometimes, the mapping from OpenAPI -> snake_case -> camelCase may produce undesirable results (think networkIP -> network_i_p, yuck, and network_ip -> networkIp sigh). We may want to override the serde field rename in a handful of cases.

We should try to keep the generated code readable, and avoid decorating each filed with a #[serde(...)] attribute.