Adds basic support for switching between Metrics and Logs in Explore.
Currently only test datasource that supports both Metrics and Logs.
Summary of changes:
* Moves mode (Metric, Logs) selection to the left of datasource
picker and add some quick styling.
* Only trigger change in ToggleButton if not selected
* Set correct mode if datasource only supports logs
Closes#16808
* devenv: working on dev env setup & dashboards
* devenv: refactored testdata app to a built in datasource instead, and moved dashboards to a devenv provisioned dashboards