XTP Schema and README update for plugin creation, changed to URLs instead of paths, added S3 support (#84)

* Add XTP plugin CLI instructions and plugin schema

* Ignore .DS_Store files on MacOS

* Rename plugin path attribute to url and add S3 support

The commit updates the plugin configuration format to use "url" instead
of "path" and adds support for loading plugins from Amazon S3. It also
improves the URL scheme handling to be more explicit and maintainable.

Key changes: - Rename path -> url in config files and code - Add S3
support via aws-sdk-s3 - Document supported URL schemes in README -
Improve URL parsing and error handling - Allow backwards compatibility
via serde alias

* Use string interpolation in error log message

* Improve error handling for S3 plugin downloads

* Simplify file URL loading in PluginService

* - Remove OciDownloader and refactor oci plugin loading to reuse the oci
Client (speedup, especially when login required) - Reuse the S3Client
(speedup) - Make the PluginService load the config, as that is the only
place it is used (moved from main.rs)

Author: joseph-wortmann

.gitignore

@@ -64,4 +64,7 @@ override.tf.json
 .terraformrc
 terraform.rc
 
+# Ignore .DS_Store files, which are created by macOS Finder
+.DS_Store
+
 # End of https://www.toptal.com/developers/gitignore/api/linux,rust,terraform