Add method getWorkflowInstance and getWorkflowInstances

[OlivierAlbertini]

🚀 Feature Proposal

Add method getWorkflowInstance and getWorkflowInstances

Motivation

In Camunda BPM you can query instances through REST API (https://docs.camunda.org/manual/7.9/reference/rest/process-instance/get/ and https://docs.camunda.org/manual/7.9/reference/rest/process-instance/get-query/), we should do it with Zeebe as well through Elasticsearch exporter (by default)

Example

In Elasticsearch, we have zeebe-record-workflow-instance index. We should query this index.

Pitch

Providing feature parity between Camunda BPM and Zeebe in order to make the transition easier.