Add preferences for chePlugin components in Devfile

[sleshchenko]

What does this PR do?

This PR adds preferences for chePlugin components in Devfile.
As an example of Devfile where this functionality is used:

Devfile with configured typesctipt plugin

specVersion: 0.0.1
name: cheTheia-in-che
projects:
  - name: devfiles
    source:
      type: git
      location: 'https://github.com/sunix/devfiles.git'
components:
  - 
    alias: theia-editor
    id: eclipse/che-theia/next
    type: cheEditor
  - 
    id: che-incubator/typescript/1.30.2
    type: chePlugin
    memoryLimit: 2048M
    preferences:
       java.jdt.ls.vmargs: '-noverify -Xmx1G -XX:+UseG1GC -XX:+UseStringDeduplication'
       java.home: '/home/user/jdk11'
commands:
  - name: 'say Hello'
    actions:
      - type: exec
        component: che-dev
        command: >
                echo "Hello!"

The same feature can be used with workspace config, plugin.{PLUGIN_PUBLISHER}/{PLUGIN_NAME}.preference.{PREFERENCE_NAME} workspace config attribute is used.

Workspace config with configured plugin

{
    "projects": [
        {
            "source": {
                "location": "https://github.com/sunix/devfiles.git",
                "type": "git",
                "parameters": {}
            },
            "name": "devfiles",
            "path": "/devfiles"
        }
    ],
    "defaultEnv": "default",
    "name": "cheTheia-in-che",
    "attributes": {
        "editor": "eclipse/che-theia/next",
        "editorComponentAlias": "theia-editor",
        "plugin.che-incubator/typescript.preference.java.jdt.ls.vmargs": "-noverify -Xmx1G -XX:+UseG1GC -XX:+UseStringDeduplication",
        "plugins": "eclipse/che-machine-exec-plugin/0.0.1,che-incubator/typescript/1.30.2",
        "sidecar.che-incubator/typescript.memory_limit": "2048M",
        "plugin.che-incubator/typescript.preference.java.home": "/home/user/jdk11"
    }
}

There is a docker image sleshchenko/che-server:plugin-prefs that contains PR's changes. It may be used for testing.

What issues does this PR fix or reference?

#12992

Release Notes

Added an ability to specify preferences for plugins in Devfile and Workspace Config.

Docs PR

Docs changes are included in the PR

[sleshchenko]

ci-test

[che-bot]

Results of automated E2E tests of Eclipse Che Multiuser on OCP:
Build details
Test report
docker image: eclipseche/che-server:13341
https://github.com/orgs/eclipse/teams/eclipse-che-qa please check this report.

[sleshchenko]

ci-test

[che-bot]

Results of automated E2E tests of Eclipse Che Multiuser on OCP:
Build details
Test report
docker image: eclipseche/che-server:13341
https://github.com/orgs/eclipse/teams/eclipse-che-qa please check this report.

[dmytro-ndp]

ci-test

[che-bot]

Results of automated E2E tests of Eclipse Che Multiuser on OCP:
Build details
Test report
docker image: eclipseche/che-server:13341
https://github.com/orgs/eclipse/teams/eclipse-che-qa please check this report.

[metlos]

Some minor comments but otherwise good!

[SkorikSergey]

Selenium tests execution on Eclipse Che Multiuser on OCP (https://ci.codenvycorp.com/job/che-pullrequests-test-ocp/1801//Selenium_20tests_20report/) doesn't show any regression against this Pull Request.

[sleshchenko]

Rebased onto master branch without any changes in PR

[sleshchenko]

ci-build