Supporting url::Url fields

[Paul-E]

The url library is a popular tool for parsing and representing URLs in Rust. It's got about 442 million downloads right now on crates.io. It's got a a parse and to_string implementation. url::Url's popularity, stable implementation, and simplicity make it a good candidate to include as a supported field type.

It seems like impl_timestamp have a macro, and something similar could be done for the types that DeriveValueTypeString supports to make it general.

[Huliiiiii]

Converted to issue #2894