Add support for HTTP proxying (#506)

* Add `--proxy` and `--no-proxy` CLI flags
* Add property `http` to `pkl:settings`
* Move `EvaluatorSettings` from `pkl:Project` to its own module and add property `http`
* Add support for proxying in server mode, and through Gradle
* Add `setProxy()` to `HttpClient`
* Add documentation

pkl-core/src/main/resources/org/pkl/core/errorMessages.properties

@@ -1053,3 +1053,7 @@ Certificates can only be loaded from `jar:` or `file:` URLs, but got:\n\
 cannotFindBuiltInCertificates=\
 Cannot find Pkl's trusted CA certificates on the class path.\n\
 To fix this problem, add dependendy `org.pkl:pkl-certs`.
+
+# suppress inspection "HttpUrlsUsage"
+malformedProxyAddress=\
+Malformed proxy URI (expecting `http://<host>[:<port>]`): `{0}`.