Add getDBProvider

[timleslie]

Ports code #5665 to pre-emptively remove dependency on legacy Keystone object.

This is the first of a few PRs which will aim to remove our usage of the Keystone object throughout the system without introducing breaking changes.

[changeset-bot]

🦋 Changeset detected

Latest commit: 3ad9853

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
@keystone-next/keystone	Patch
@keystone-next/types	Patch
@keystone-next/test-utils-legacy	Patch

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