Seamless Timestamp/Date encoding/decoding

### Is your feature request related to a problem? Please describe.
Currently dealing with `google.protobuf.Timestamp` types is rather clunky if you want to interop native JS `Date` instances. Custom code must be used upon reading and writing messages to translate.

### Describe the solution you'd like
Introduce an option similar to `longs` and `bytes` which enables the generated code _and_ types to prefer `Date` instances for `Timestamp`-typed fields.

Implementation-wise my suggestion is as follows:
- Add a new option called `timestamp` (or `google.protobuf.Timestamp`) which accepts the value `Date`
- When enabled, the generated types would use `Date` for any timestamp fields
- When enabled, the `protobufjs`-wrapping code would override the default `toObject` and `fromObject` on the `Timestamp` class to accept dates

If/when `protobufjs` gains an option to do native Dates, the third bullet could be replaced with that (or I could try and get something landed with them first, before pursing the type-generation change here).

I'm happy to implement this myself and submit a PR, provided I a bit of steer on whether this would go anywhere.

### Describe alternatives you've considered

I note that this was previously asked for in https://github.com/grpc/grpc-node/issues/357 and closed, but the ecosystem has progressed since then.

I've done a bunch of code spelunking today and here's what I think the current state of things is:

- protobufjs v6 gained support for custom wrappers, which can extend the default `toObject`/`fromObject` implementations (mentioned in https://github.com/protobufjs/protobuf.js/issues/677)
- the custom wrappers are defined on the `protobufjs` module globally, and aren't directly exposed by `proto-loader`
- it is also possible to implement by augmenting the methods of the generated `Timestamp` class, but this also isn't directly exposed by `proto-loader` (they're on the protobufjs root).
- There is an old protobuf.js PR to add a wrapper native Date support, but this appears to have stalled and it's focused on JSON encoding anyway [https://github.com/protobufjs/protobuf.js/pull/1258]
- Even if that PR was responded to by maintainers and landed, returning `Date`s from toObject would likely need to be opt-in to keep remain backwards compatible
- `proto-loader`'s typescript generation code would also need a flag to modify the generated types, `fromObject` could be backwards compatible and accept either the `seconds/nsec` object or `Date`, but `toObject` has to pick a side.

If there's another way to make this all interop nicely with less work, or without making changes to core I'd be happy to take that approach instead!