[Feature Request] Add skipped_fields field constraint for finer-grained skipped behavior

[rodaine]

Feature description:

Today, we support the skipped standard constraint on message fields to skip any validation constraints applied to the underlying type. This is currently an all or nothing scenario, when perhaps it's desired to opt-out of one or a few of the message's fields.

Potentially, we can support this via a skipped_fields constraint which takes a FieldMask and can specify which not to evaluate.

Problem it solves or use case:

Example use-case where a message describing a resource elided a constraint for a field and had the RPC request object use a CEL expression to validate that nested field.

Proposed implementation or solution:

# validate.proto

# <snip>

message FieldConstraints {
  # <snip>
  oneof skip {
    bool skipped = 24;
    google.protobuf.FieldMask skipped_fields = XX;
  }
  # <snip>
}

# <snip>

# example.proto

message Foo {
  string id   = 1 [(buf.validate.field).string.uuid = true];
  bytes  data = 2 [(buf.validate.field).bytes.min_len = 42];
}

message Bar {
  Foo foo = 1 [(buf.validate.field).skipped_fields.paths = "id" ];
}

In the example above, Foo by itself will verify the value of the id field to be a UUID and the data field to be of a minimum length. However, if validate is called on a Bar containing a Foo, only the data field will be evaluated as id has been skipped.