Make provider the bundle key of workspaces (or document why not)

Problem/Motivation

Aftermath of #3095414: Add the concept of workspace providers → :
Last nicght it occured to me that design-pattern-wise, the added workspace.provider base field should really be the bundle key.

This might solve the question "where do providers store their custom payload" elegantly as "in bundle fields".

Setting critical as considering this after beta might have unforseeable costs.

Proposed resolution

- Make provider the bundle key of workspaces (or document why not)

Thoughts:
- I can't think of another reasonable use case for bundle.
- If noone uses that key, no harm is done.
- It selves the EntityCreateAccess problem very elegantly, by using the existing mandatory bundle parameter
- What is the planned upgrade path to populate the "provider" field?
(Maybe it simplifies that upgrade path if the DefaultWorkspaceProvider ID is "workspace")

Remaining tasks

Consider, do it.