[WIPTheia] Add support for preferences schema from contribution point of a manifest file

[akurinnoy]

Description

A plugin should be able to provide its own preferences schema. We should implement a mechanism which parses and respects the scheme provided by a plugin.

1. Describe the schema in json file
2. Point the file location in package.json [TODO define the configuration in package.json]
3. Plugin meta data handles the value in package json, proceses and passes along with other plugin metadata

[AndrienkoAleksandr]

https://code.visualstudio.com/docs/extensionAPI/extension-points#_contributesconfiguration

[AndrienkoAleksandr]

I'm going to PTO, so summary about current state:
Working branch: https://github.com/theia-ide/theia/tree/CHE-9990-2
Samples (in dirty development state): https://github.com/theia-demo-plugins/theia-frontend-plugin/tree/workspaceConfigSamples;
Done: parsed preference chema from plugin contribution point in the manifest file, created consolidated configuration for extensions and plugins based on extension pref chema and plugin pref chema.
Remaining stuff:

• throw onConfigurationChangedEvent,
• update configuration on uninstall extension,
• apply to the workspace configuration "user preferences",
• check synchronization for all pref events,
• apply more samples and clean up them.
  During development created issue:
  Implement fields "readonly [key: string]: any;" for WorkspaceConfiguration in the plugin api eclipse-theia/theia#2339
  Plugin api: theia#workspace#getConfiguration doesn't handle resource argument. eclipse-theia/theia#2290
  [plug-in] Implement inspect configuration eclipse-theia/theia#2289