Update Schema Architecture/Design

[zFernand0]

Here is a list of possible requirements based on recent discussions:

• Allow schema to be updated simultaneously by different clients
• Avoid replacing global schema with just the contents of ~/.zowe/plugins/plugins.json
  • ZE may need to maintain a list of registered/installed extensions
    • Possibly enhance the plugins.json file to allow VSCEs to be added, OR
    • Add the ability for VSCEs to create and maintain an equivalent extensions.json file
• Handle possible differences between CLI plugin schema and VSCE schema definitions
  • Example: ftp-for-cli may be different than what's expected by ftp-for-ze
• No need to add the ability to update project schema files at this time.

Originally posted by @zFernand0 in zowe/zowe-explorer-vscode#1642 (comment)