Dashboard: Replace datasource name with a reference object

[ryantxu]

Fixes #1664, See also #32853

We currently store the datasource name in the dashboard, this has a few bad effects:

1. not enough information in a dashboard to know if the queries are valid without a database lookup. See Make it possible to validate dashboard JSON with datasource queries #33761
2. renaming a datasource will break all references

This replaces our existing string object with a datasourceRef:

export interface DatasourceRef {
  type?: string;
  uid?: string;
}

Although we could simply replace our existing name with the uid, using an object with type makes defining dashboards programatically MUCH more natural. Specifically, this allows us to day "I need a loki datasource, but I don't care which one". Also the type parameter will allow validating the query syntax without without needing to lookup uid > datasource.pluginType

TODO:

• migrate variable queries
• dashboard queries...
• Unified alerting seems to be working and tests pass (after a code update by torkel, se changes in expr/graph and expr/nodes)
• Updated legacy alerting alert rule extractor
• What should dashboard exporter do?
• Annotation queries

[torkelo]

This looks very promising, awesome job

[ryantxu]

cc @sdboyer @malcolmholmes @ying-jeanne

[malcolmholmes]

I assume that this introduces a new schema version, and that older schema versions will be migrated to this version. In which case, at least in this sense, this isn't a breaking change. Existing dashboard json will continue to work. It will only break if you update the schema version without updating your dashboard references.

Are these assumptions correct?

[hugohaggmark]

@malcolmholmes this changes the schemaVersion

[dginhoux]

Very great idea, thank you very mich for this improvment

[marefr]

I assume that this introduces a new schema version, and that older schema versions will be migrated to this version. In which case, at least in this sense, this isn't a breaking change. Existing dashboard json will continue to work. It will only break if you update the schema version without updating your dashboard references.

Are these assumptions correct?

I removed the breaking change label for now.