AdhocFilters: Support `-- Mixed --` data source

### Discussed in https://github.com/grafana/grafana/discussions/39396 <div type='discussions-op-text'> ^{Originally posted by **Grubana** September 19, 2021} <!-- Please use this template to create your bug report. By providing as much info as possible you help us understand the issue, reproduce it and resolve it for you quicker. Therefor take a couple of extra minutes to make sure you have provided all info needed. PROTIP: record your screen and attach it as a gif to showcase the issue. - Questions should be posted to: https://community.grafana.com - Use query inspector to troubleshoot issues: https://bit.ly/2XNF6YS - How to record and attach gif: https://bit.ly/2Mi8T6K --> **What happened**:  When adding an adhoc filter within a mixed datasource panel, the filter is not getting applied to the queries. **What you expected to happen**: Filters are applied to all adhoc enabled datasources within the panel **How to reproduce it (as minimally and precisely as possible)**: Create a table panel with a mixed datasource. Try to filter for a value within the table. **Anything else we need to know?**: Looks like the adhoc filter is tied to the mixed datasource type, instead of the concrete datasources used in the panel config **Environment**: - Grafana version: v8.1.3 (a61f38238c) </div>