protoc-gen-ent: support timestamp and date

[simon-wenmouth]

Use of types google.protobuf.Timestamp and google.type.Date currently panics because they are MessageType but not edges.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x30 pc=0xae904d]

goroutine 1 [running]:
main.toEdge(0xc0003d1ba0)
... entgo.io/contrib@v0.5.0/entproto/cmd/protoc-gen-ent/main.go:146 +0x8d

The method isEdge was updated to recognize these types as fields, and the method toField was updated to map them as field.Time.

[simon-wenmouth]

The tool protoc-gen-ent does not currently have support for time fields. I added mappings for google.protobuf.timestamp and google.type.date because they have a mapping to field.Time. I did not add a mapping for google.type.timeofday (e.g. SQL time) because ent does not currently support these "civil" date/time types. If that is of interest, I'm willing to implement this feature and discuss the design with your team. At a high level I expect this would mean adding field.CivilDate, field.CivilTime and field.CivilDateTime with the underlying structs coming from either cloud.google.com/go/civil or golang-sql/civil, with corresponding updates to protoc-gen-ent afterwards.

I think this PR duplicates in part the work in #447 (March 2023), which has not received feedback.

I welcome your feedback, and look forward to the possibility of the feature getting merged in the future.

Thanks!