[multi-provider] Move multi-provider into SDK

The multi-provider in the contrib repo should be moved into the SDK.

Requirements:

• multi-provider is moved from contribs into SDK
• multi-provider deprecated/removed in the contribs (@toddbaert @beeme1mr or @aepfli can help with this)
• optionally, there may be simplifications than can be done once the multi-provider has access to SDK internals

Note:

• the multi-provider should be marked as experimental for now, using in-line docs.