Add serde feature to allow reuse of some value types directly.

[didoloan]

Hello,

It would be very useful to have serde's Serialize and Deserialize traits derived for some Cql value types like CqlDuration and Counter.

I could make a pull request with an implementation. I would would like to hear other opinions on this. Maybe I'm the only one seeing this as an issue.

[Lorak-mmk]

Related: #748

[Sokom141]

I second this.
It would be really useful to use serde on these types. Unfortunately the derive for the remote types as discussed in #748 is not always the most ergonomic thing to do (for example if you do it on a type that is also wrapped in an Option) so the best thing would be having this feature available.