Change CLI flag and Gradle name for proxy options (#526)

Rationale: "proxy" can mean very different things (e.g. java.lang.reflect.Proxy in Java).
This makes the flag name more specific.

CLI:
* `--proxy` -> `--http-proxy`
* `--no-proxy` -> `--http-no-proxy`

Gradle:
* `proxyAddress` -> `httpProxy`
* `noProxy` -> `httpNoProxy`

docs/modules/pkl-cli/partials/cli-common-options.adoc

@@ -123,7 +123,7 @@ Certificates need to be X.509 certificates in PEM format.
 For other methods of configuring certificates, see xref:pkl-cli:index.adoc#ca-certs[CA Certificates].
 ====
 
-.--proxy
+.--http-proxy
 [%collapsible]
 ====
 Default: (none) +
@@ -132,7 +132,7 @@ Configures HTTP connections to connect to the provided proxy address.
 The URI must have scheme `http`, and may not contain anything other than a host and port.
 ====
 
-.--no-proxy
+.--http-no-proxy
 [%collapsible]
 ====
 Default: (none) +