Create Server endpoints (v2) returning the checkpoins data

[benedeki]

Background

We need to read all the measurements across a checkpoint of certain name within a flow.

Feature

Create the REST API endpoint and required class in model module to return the data received from DB.

Proposed Solution [Optional]

Use #141 for endpoints path.
Envelope is not needed, to be implemented yet.

Depends on #187

[benedeki]

This item depends on:

• Create DB function to get all checkpoint data of a flow #187
  By Dependent Issues.