Add custom_scalar tag as a extension point

[Guiguiprim]

I'm not sure if you will be interested by this PR but I will try my luck as I feel like other might use it (otherwise I will keep it on a fork).

Context: For performance and memory usage reasons I wanted to optimise the data structures I use with prost. In my use-case I wanted to use a string interner for some fields and arrays instead of bytes for others, but this could also be used for small string optimisation or tiny vec and more.

To do it in a generic way, I added:

• a trait CustomScalarInterface
• support for a custom_scalar(interface_path) tag in prost-build and prost-derive

This make it possible to implement as a crate user any scalar wrapping type without needing to touch prost. It is an expert API as the user would need to correctly implement CustomScalarInterface in a way that is compatible with the declared protobuf type.

[Guiguiprim]

I just realized this is the same idea as #1230.

I went for a trait instead of a module (as I almost done at first) as I thought it would make the feature easier to use and understand.

I also added support in prost-build which the other did not I believe.

Otherwise naming can obviously be redone if a better one is proposed.