Rename repo:assetId to Resolve repo:id

## What are the schemas that are affected by the issue
This affects schemas/external/repo/asset.schema.json and schemas/external/repo/common.schema.json
In the assets schema an assetID is defined with the repo: prefix. Several discussions pointed out that this ID is too narrowly referring to assets and other repository-managed resources also need an id. A new property should be defined in the common.schema.json

The semantics if this property is very similar to repo:assetID:
- Assigned by the repo
- doesn't change over the lifetime of the resource as long as it is stored in the same repository.
- doesn't transfer with the resource when the resource is copied or moved to another repository service
- does not have semantic outside the repository and SHOULD not be used to externally link resources to each other when they exist outside the repository. For example, export to a different media system renders that ID useless as a link property.
- can be used in the repository that assigned the value to uniquely identity a resource.

Start by comparing the repo:assetID definition in schemas/external/repo/asset.schema.json with the shared definitions in schemas/external/repo/common.schema.json. Define the generalized repo:id property in common.schema.json and update the affected asset schema as needed; done means the schemas capture the stated repository-scoped identity semantics without limiting the property to assets.

Written by the indexing model from the issue text.