feat: experimental in-memory files cache with eviction policy support (…

…static-web-server#328)

Via a new advanced configuration entry.

The feature also supports expiration policies such as Time To Live (TTL) and Time To Idle (TTI).

Admission to a cache is controlled by the Least Frequently Used (LFU) policy and the eviction from a cache is controlled by the Least Recently Used (LRU) policy.

Example:

```toml
[general]

[advanced]

[advanced.memory-cache]
# Maximum capacity entries of the memory cache-store. Default 256
capacity = 256
# Time to live in seconds of a cached file entry. Default 1h
ttl = 3600
# Time to idle in seconds of a cached file entry. Default 5min
tti = 300
# Maximum size in bytes for a file entry to be cached. Default 8MB
max-file-size = 8192
```

Note that this feature requires Rust 1.76.0 or newer.

This feature is **experimental**. However, when stabilized then a cargo feature will be available as well as a proper documentation page.

.github/workflows/devel.yml

@@ -72,7 +72,7 @@ jobs:
           # We test against the latest and minimum Rust stable version.
           - build: pinned
             os: ubuntu-22.04
-            rust: 1.74.0
+            rust: 1.76.0
           # Some of our release builds are generated by a nightly compiler to take
           # advantage of the latest optimizations/compile time improvements.
           - build: linux-musl