Keep serialized protobuf binaries for testing

[kzys]

These serialized binaries can be used to test changes like #4422.

[kzys]

Hmm, this may not work.

https://protobuf.dev/programming-guides/encoding/#implications

• Do not assume the byte output of a serialized message is stable. This is especially true for messages with transitive bytes fields representing other serialized protocol buffer messages.
• By default, repeated invocations of serialization methods on the same protocol buffer message instance may not produce the same byte output. That is, the default serialization is not deterministic.

[tonistiigi]

Deterministic encoding is needed for LLB encoding(at least for same version of buildkit) as the format is content-addressable and uses checksums of encoded messages are used for deduplication.

[kzys]

Let me set Deterministic on MarshalOptions. This may be the best we could do here.

	// Deterministic controls whether the same message will always be
	// serialized to the same bytes within the same binary.
	//
	// Setting this option guarantees that repeated serialization of
	// the same message will return the same bytes, and that different
	// processes of the same binary (which may be executing on different
	// machines) will serialize equal messages to the same bytes.
	// It has no effect on the resulting size of the encoded message compared
	// to a non-deterministic marshal.
	//
	// Note that the deterministic serialization is NOT canonical across
	// languages. It is not guaranteed to remain stable over time. It is
	// unstable across different builds with schema changes due to unknown
	// fields. Users who need canonical serialization (e.g., persistent
	// storage in a canonical form, fingerprinting, etc.) must define
	// their own canonicalization specification and implement their own
	// serializer rather than relying on this API.
	//
	// If deterministic serialization is requested, map entries will be
	// sorted by keys in lexographical order. This is an implementation
	// detail and subject to change.
	Deterministic [bool](https://pkg.go.dev/builtin#bool)

[kzys]

MarshalOptions is only available in google.golang.org/protobuf, which can't handle existing gogo-generated protobuf messages.

gogo's equivalent is stable_marshaler_all, which we already have in LLB-related files such as solver/pb/ops.proto.