v0.6.3

This is release for our new server functions rewrite and Axum 0.7 support.
This should be a relatively feature-rich release, with limited breaking changes.

## Migration

### Actix

- You can remove any explicit `.handle_server_fns()` call in your `main.rs`, as server functions are now handled in `.leptos_routes()`
- The current `extract` function has been removed, and replaced with a new `extract` that has the same API as the current [`extractor`](https://docs.rs/leptos_actix/latest/leptos_actix/fn.extractor.html). I think this API is strictly better, but please share feedback if you disagree.
 
### Axum
- This release supports Axum 0.7, so you'll need to migrate from Axum 0.6. The easiest way to do this is probably to consult [the diff on one of the examples](https://github.com/leptos-rs/leptos/pull/2158/files#diff-ceceec2ecd620c6891344b9407c391c6a9ca3b2d2f101881cd09ddd7f5a31981). (Note that along with Axum 0.7, you'll need to update to `http` 1.0 and `tower_http` 0.5.)
- You can remove any explicit `.handle_server_fns()` call in your `main.rs`, as server functions are now handled in `.leptos_routes()`
- The current `extract` function has been removed, and replaced with a new `extract` that has the same API as the current [`extractor`](https://docs.rs/leptos_axum/latest/leptos_axum/fn.extractor.html). I think this API is strictly better, but please share feedback if you disagree.
- `RequestParts` has been removed, as `http::request::Parts` now implements `Clone`: any `use_context::<RequestParts>()` should be updated to use `Parts` directly instead.

### `ServerFnError::new()`
The addition of custom error types means that constructing `ServerFnError` inside server functions can cause type inference errors:
```rust
let oops = Err(ServerFnError::ServerError("No server state".to_string()));
return oops;  // this is fine
oops? // this is not: cannot infer type of the type parameter `E` declared on the enum `ServerFnError`
```
As a result, we've added a helper `ServerFnError::new` which simply constructs a `ServerFnError::<NoCustomError>::ServerError`:
```rust
let oops = Err(ServerFnError::new("No server state"));
return oops;  // this is fine
oops? // this is also fine now
```
This has the benefit of being more concise than the earlier pattern in any case.

## Features

A rewritten server function system that is backwards-compatible, but reduces binary size and increases flexibility, specifically by allowing
  - automatic setup of server fn handlers with `.leptos_routes()` from the integrations
  - a variety of additional built-in encodings (rkyv, multipart forms/file uploads) in addition to the current set (GET URL, POST URL, CBOR, Rkyv) (#1868, #1989)
  - support for streaming responses from server functions (#1284)
  - ability to create custom user encodings for input and output by deriving `IntoReq`, `FromReq`, `IntoRes`, and/or `FromRes` traits
  - ability to mix and match encodings easily: This server function should be a JSON POST request and a ByteStream response, this one should be GET URL request and an rkyv response, etc.; any combination of the encodings above is supported
  - custom error types (#1657)
  - a `#[middleware]` macro to add per-server-function middleware from the Tower or Actix ecosystems (#1461)

Note: The additional included encodings (`serde_lite`, `rkyv`, multipart form data) are all enabled by additive features on the `server_fn` crate. If you want to use them you can just add that crate as a dependency and enable the required features.

**Example**: You can find a comprehensive example of these new features in the new [`server_fns_axum`](https://github.com/leptos-rs/leptos/blob/leptos_v0.6/examples/server_fns_axum/src/app.rs) example.

**Full Changelog**: https://github.com/leptos-rs/leptos/compare/v0.5.7...0.6.3