Allow validation of "string-like" types

[cortopy]

I have code for protobuf messages generate with prost, which will be something like

struct Address {
  line1: prost::alloc::string::String
}

for most purposes line1 is just a String and any function that works with one will work with the other.

However, because of the following function

validator/validator_derive/src/asserts.rs

Line 72 in 7e7ba65

pub fn assert_string_type(name: &str, type_name: &str, field_type: &syn::Type) {

I can't use the derive annotations

Would it be acceptable to write the check as a match for "String"?

[loyd]

• also for CompactString and other strings with SSO

[Keats]

This would be fixed by #205