Replace url::Url with our own type

[heaths]

Before we GA, I would like to replace the url::Url type that we re-export from typespec_client_core and azure_core with our own type that is better optimized for scenarios we need e.g.,

1. Mutable query parameters. Our generated client code mutates URLs enough that taking an owned iterator just to reserialize the entire URL later becomes expensive over time.
2. Ability to construct from a &str for which we are reasonably sure it's a URL e.g., returned resource IDs. This could be exposed via From<&str> or a new_unchecked constructor.
3. Considering the ability to construct from a &str parts could be retrieved as Cow<'_, str> and, thus, we'd implement ToOwned and friends.

The scenarios all this enables are:

1. Ability to mutate parameters more with less CPU time and likely no significant memory overhead (we should measure speed and allocations; see https://github.com/heaths/ordinal-rs/pull/13/files for one possibility).
2. Ability to return cheap &str slices for parts within a URL.

[heaths]

Initially, I imagine some structure like this (not necessarily sound - just some high-level ideating):

#[derive(Default)] // ...
pub struct Url<'a>(private::Url<'a>);

impl<'a> Url<'a> {
  pub fn new() -> Self {
    Default::default()
  }
  pub fn new_unchecked(url: &'a str) -> Self {
    Self(private::Url::Owned(url))
  }
}

// impl From, FromStr, ToOwned, AsRef, etc.

mod private {
  #[derive(Debug, Default)] // ...
  pub enum Url<'a> {
    Borrowed(&'a str),
    #[default]
    Owned {
      value: String,
      params: HashMap<String, Cow<'a, str>>,
      fragment: Option<String>,
    }
  }
}

/cc @analogrelay @LarryOsterman

[heaths]

Alternatively, we newtype url::Url for now and expose only limited methods we think customers would use normally. If they want more from url::Url, we'll support impl From<url::Url> for Url and impl From<Url> for url::Url and they can get out of what they need. But this means if we do ever replace the 3P dependency completely¹ we'd still have to support that conversion.

Footnotes

1. Assuming that we even can remove it. Right now, cargo tree -i url shows it's only used by our crates, http-types - which we want to remove in Replace http-types with http #2233 - and fe2o3-amqp which is only used for azure_core_amqp-based crates, which will only be a couple. ↩