Introduce schema-version for element templates

What should we do?

Currently, element templates don’t define a schema or schema-version that they adhere to.

This creates challenges:

• When we are changing element templates, these might not work in older Modeler version. Hence when loading these element templates, the Camunda Modeler will just throw a generic error message.

However, a solution is already available: the https://github.com/camunda/element-templates-json-schema repo defines a versioned schema, which can be used to…

• verify the validity of a given element template
• ensure that only element templates, which are supported in the current Camunda Modeler version, are loaded.

This issue proposes the following changes:

Prio 1

• Add https://github.com/camunda/element-templates-json-schema as dependency
• Element template schema json shall allow multiple element templates in an array as valid json (https://github.com/camunda/element-templates-json-schema/issues/15)
• Release + integrate a version 0.3.0 of https://github.com/camunda/element-templates-json-schema, which includes the element templates changes introduced here: https://github.com/camunda/camunda-modeler/issues/2070
• Update element templates to include a new field: "$schema": "https://unpkg.com/@camunda/element-templates-json-schema@0.2.0/resources/schema.json" (https://github.com/camunda/camunda-modeler/pull/2102)
• Explicitly define in the codeBase, which element-template versions are supported (https://github.com/bpmn-io/bpmn-js-properties-panel/pull/431)
• When loading an element template, check the version compatability. If incompatabile, ignore the respective element template giving a meaningful notifaction (ala. “Vx.y.z of element templates is not supported, please update your Camunda Modeler to the latest version”). With the first release, support version 0.2.0 and yet to be released version 0.3.0 (https://github.com/bpmn-io/bpmn-js-properties-panel/pull/431)
• Update the element template docs (https://github.com/camunda/camunda-modeler/issues/2161)

Prio 2

• Validate the element template before applying it using https://github.com/bpmn-io/json-schema-validator and print the error to the user (if any) (Will be tracked in https://github.com/camunda/camunda-modeler/issues/2159)

Example

[
  {
    "$schema": "https://unpkg.com/@camunda/element-templates-json-schema@0.2.0/resources/schema.json",    
    "name": "Template 1",
    "id": "sometemplate",
    "version": 1,
    "appliesTo": [
      "bpmn:ServiceTask"
    ],
    "properties": [
      ...
    ]
  },
  {
    "name": "Template 2",
    ...
  }
]

Why should we do it?

This change will help us to manage changes in element templates in a more user-friendly way.

[MaxTru]

Only thing missing: update of docs

[MaxTru]

Did final smoke test including:

• Unsupported schemaVersion shall be ignored
• Tested old scopes array design for camunda:Connector
• Tested new scopes object design for camunda:errorEventDefinitions

Great job everyone! Closing this issue (as @nikku stated, documentation is tracked seperately)