Add endpoint to get beamline parameters

The first version of the config server just has one endpoint to get any whitelisted file either as a string, dict, or raw bytes.

We should make the server handle common configurations more intelligently. One of the most widely used configuration files is `beamlineParameters`. We should add another endpoint to get this specific file. It should require a beamline parameter and work out the file location from this. It should also have an option to specify a specific parameter. If this isn't used, then just send over the entire beamlineParameter dict.

The old version of this repo had similar functionality - see https://github.com/DiamondLightSource/daq-config-server/blob/4b0a2e2e8d395381645a653ec31109181738d1be/src/daq_config_server/app.py

## Acceptance Criteria
- Create + test new `get_beamline_parameter` endpoint which matches the above description