fix!: make host optional

This PR fixes the current implementation considering
a host is mandatory in the Url. Yet this is incorrect
as some schema don't require a host (ex: `file://`).
This fix introduce a breaking change both for the Rust
crate and the Python package.

Author: qjerome

faup/src/grammar.pest

@@ -1,4 +1,4 @@
-url    = ${ SOI ~ scheme ~ "://" ~ userinfo? ~ host ~ (":" ~ port)? ~ path? ~ query? ~ fragment? ~ EOI }
+url    = ${ SOI ~ scheme ~ "://" ~ userinfo? ~ host? ~ (":" ~ port)? ~ path? ~ query? ~ fragment? ~ EOI }
 scheme =  { ASCII_ALPHANUMERIC+ }
 
 username = ${ (!(":" | "@" | "/" | "?" | "#" | "[" | "]" | WHITE_SPACE) ~ ANY)+ }