json: emit unbroken base64 for bytes fields ({nowrap = true})

Canonical proto3 JSON expects RFC 4648 unwrapped base64, but Tarantool's
`digest.base64_encode` defaults to RFC 2045 MIME-style 76-char line
wrapping. Any `bytes` payload past ~57 bytes used to land in the JSON
string with an embedded `\n`, which breaks every spec-compliant
consumer (grpc-gateway, protojson, protobuf-go's JSON, ...).

Pass `{nowrap = true}` at the two encode sites: the per-field bytes
encoder and the `google.protobuf.Any` opaque-fallback `value` encoder.

Surfaced by tarantool-etcd's `TestJSONGatewayBytesUnwrapped` over Range
responses whose `value` is >=57 bytes; its reference grpc-gateway never
emits the wrapped form.

Initial commit: protoc-gen-tarantool plugin + pb runtime

A protoc plugin (Go) and a pure-Lua + LuaJIT-FFI runtime that give
Tarantool a complete proto3 + gRPC stack. Two codegen modes (full
inline / runtime descriptor), 226-test luatest suite, 18-fixture
mainline-protoc interop corpus, JSON codec, well-known types,
gRPC client/server factories, runtime .proto parser, microbench
harness with allocation regression gate.

Covers PLAN.md M1-M5. Module is `pb` (not `protobuf`) to avoid
colliding with Tarantool's built-in encode-only `protobuf` module.