Add optional metadata to BatchUpdateLocationCapacity #214

Verification

• I have searched the existing issues to ensure this feature has not already been requested.

Provide a detailed description of the proposed feature.

The GB consumer stores capacity_no_degradation_kw in location metadata, and it changes about every three months. The BatchUpdateLocationCapacity route can't set metadata, so those updates still need UpdateLocation for every location (#199).

What is the motivation for the feature?

Required for openclimatefix/solar-consumer#261

Outline a potential approach or solution (optional).

Could we add an optional new_metadata to each update, keeping the existing metadata when it's not set?