test: add property-based tests for new proxy features

Add proptest-based tests covering content_type_for, cache_control_for,
backend_addr, UUID v4 request IDs, rewrite rule compilation, body
rewriting, config TOML deserialization, path traversal prevention,
metrics label validation, and static file serving.

Signed-off-by: Sienna Meridian Satterwhite <sienna@sunbeam.pt>

Cargo.toml

@@ -73,6 +73,8 @@ libc        = "0.2"
 
 [dev-dependencies]
 criterion = { version = "0.5", features = ["html_reports"] }
+proptest  = "1"
+tempfile  = "3"
 
 [[bench]]
 name = "scanner_bench"