AssetBeds should be unique together by Asset, Asset Type and Bed

[rithviknishad]

Describe the bug
Currently, we are able to link multiple assets of the same type (say multiple HL7 monitors) to the same bed.

To Reproduce
Steps to reproduce the behavior:

1. Create a bed
2. Create two assets of type: HL7 Monitors
3. Configure the two assets to use the created bed.

Expected behavior
The backend should raise a validation error to prevent linking multiple assets of same asset type to a single bed.

[rithviknishad]

We can't have this validation for camera's as each bed preset creates new AssetBed record.

We might also need to restrict editing asset_type once an asset is created so that a Camera asset with multiple beds should never be updated to be a monitor as it shouldn't be linked with multiple bed.

cc: @gigincg @vigneshhari