Make datasource UID editable in GUI

### Discussed in https://github.com/grafana/grafana/discussions/45230 <div type='discussions-op-text'> ^{Originally posted by **XANi** February 9, 2022} Previously, as long as 2 grafana instances (say dev and prod, or dc1 and dc2) had identically named data sources it was relatively straightforward to either export/import dashboards between them manually or automate that. That was hacky (it relied on 2 instances having same name for data source), and caused problems with renaming ( #1664 https://github.com/grafana/grafana/pull/33817 ) but *generally* worked. But in recent version exported dashboard JSON now contains UID. Now name of the data source is user-controllable but UID is not, which means only way for new instance to accept dashboards exported from old instance is either: * set up provisioning with same UIDs (which is fine solution if you're already automated your deployments but otherwise "too late") * editing database to match that UID (and as I understand break any existing dashboards in the database) * editing dashboard JSONs to use legacy by-name method It would be useful if there was an option to change the datasouce UID, either in web config (altho that would possibly require migrating existing ones), or to have it as option during export. </div>