Feature request [2.0]: Query + Map dashboard variable type

**Proposal:**
Create a dashboard variable type that uses columns in query results to create variable key-value mappings. It could expect specific column labels (`key` and `value`) or allow users to select which columns to use as keys and values in the UI.

**Use case:**
This would allow users to dynamically generate map variables from query results.

**Example:**
The example below would map bucket names to bucketIDs. Bucket IDs are static, but users can change bucket names. Using this to populate a `bucket` variable would make the list of buckets human-readable, but populate dashboard queries with the bucketID. Bucket names could be changed without breaking anything as long as queries use `from()`'s `bucketID` parameter.

```js
buckets()
|> map(fn: (r) => ({ key: r.name, value: r.id }))
```

Start by reviewing the dashboard variable UI and the query-result handling described in the proposal. Resolve whether mappings use fixed `key` and `value` columns or user-selected columns, then verify that query results provide human-readable labels while dashboard queries use the underlying values.

Written by the indexing model from the issue text.