Add Release published timestamp field to .proto

[elfenlaid]

Part of hexpm/hex#1113

Specifically hexpm/hex#1113 (comment)

---

The idea is to replicate what hexpm shows for package release published timestamps in its web interface https://github.com/hexpm/hexpm/blob/78e8f9665992de71b22571c7d09fb738fffdd5f3/lib/hexpm_web/templates/package/versions.html.eex#L7

This published_at field is then going to be used to display a package release published dates in mix hex.outdated and for package cooldown checks described in this proposal https://gist.github.com/ericmj/16488f164ca2045e12f0f79a73c45031

[elfenlaid]

The exact same type lives in the neighbour file

hex_core/proto/hex_pb_names.proto

Lines 17 to 24 in 795c08e

	optional Timestamp updated_at = 3;
	}
	// Based on google.protobuf.Timestamp
	// https://github.com/protocolbuffers/protobuf/blob/v3.15.8/src/google/protobuf/timestamp.proto#L136:L147
	message Timestamp {
	required int64 seconds = 1;
	required int32 nanos = 2;

Yet, I've decided against sharing this type as I think it would be easier to replace it with a native protobuf Timestamp available in the third version of protobuf.

[elfenlaid]

Dear reviewers, please pay attention to this diff.

I'm not sure why these checks are dropped and it's possibly an error.

I believe it's related to gdb_opts from

hex_core/rebar.config

Lines 7 to 17 in 22992eb

	{gpb_opts, [
	{i, "proto"},
	{o_erl, "src"},
	{o_hrl, "src"},
	{verify, always},
	{strings_as_binaries, true},
	{maps, true},
	{maps_unset_optional, omitted},
	{report_warnings, true},
	{target_erlang_version, 17}
	]}.

or to my Erlang version: Erlang/OTP 27 [erts-15.2.7.4]

[ericmj]

Thank you! 💜