Add `derive(Eq)` for eligible google types

[lpzantony]

Hello,

I noticed that in pbjson-types's google.protobuf.rs, google types only get the trait PartialEq, not Eq. Which make impossible to use thes types in a BTreeSet/Map because they require Ord and thus Eq.

I know that types using floats/double can't use trait Eq, but types like Timestamp which only uses integers, should have no problem using derive(Eq).

Any chance you can add it to non-float google types ?