Implement Count Endpoints

[MRichards99]

Description:
This covers the following endpoints:

• GET /datasets/{pid}/files/count
• GET /datasets/count
• GET /documents/count
• GET /instruments/count

Each of these endpoints only accepts the where as a query parameter (rather than filter) so only the WHERE filter is supported here. This should be implemented elsewhere but should be noted as a reminder. The following format should be used as a response which is slightly different to DataGateway API:

{
    "count": 15
}

Acceptance criteria:

• Implement each endpoint
• Write test cases to cover each endpoint