`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 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