`CacheDirective` enum is missing some directives

## Expected Behavior
I should be able to use `CacheDirective::Immutable` for implementing `Cache-Control` headers for the cache-busting pattern.

## Current Behavior
I have to use `CacheDirective::Extension("immutable".to_owned(), None)`

## Possible Solution
Add `CacheDirective` variants for the following variants [listed on MDN](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control) but missing from the enum:

* must-understand
* immutable
* stale-while-revalidate
* stale-if-error

## Your Environment

- Rust Version (I.e, output of `rustc -V`): rustc 1.59.0 (9d1b2106e 2022-02-23)
- Actix Web Version: 4.0.1