Add `/cache/dcm-tags` API to retrieve what data is available in the Orthanc index and Sonador resource cache

API needs to:

• Provide a list of the DICOM tags that are currently available for the cache. (These should be read from the main configuration and presented alongside the resource with which they are associated.)
• Specify the value representation (VR) of the tag.

Example response (one tag):

{
  Patient: {
    '0010,0020': {
      'tag': 'PatientID',
      'name': 'Patient ID',
      'vr': {
        'code': 'LO',
        'description': 'Long String',
      }
    }
  },
  Study: {
    ...
  },
  Series: {
    ...
  },
  Instance: {
    ...
  }
}

Development tasks:

• Add API endpoint to Orthanc
• Add support to Sonador so that it is able to approve requests to the dcm-tags endpoint for non-admin users. (Should be granted with the query API permission.)