refactor: use shared reqwest::Client across all registry handlers

Add http_client field to AppState, initialized once at startup.
Replace per-request Client::builder() calls in npm, maven, pypi,
and docker registry handlers with the shared instance.
This reuses the connection pool across requests instead of
creating a new client on every proxy fetch.

Bump version to 0.2.20.

Cargo.toml

@@ -7,7 +7,7 @@ members = [
 ]
 
 [workspace.package]
-version = "0.2.19"
+version = "0.2.20"
 edition = "2021"
 license = "MIT"
 authors = ["DevITWay <devitway@gmail.com>"]