[ENHANCEMENT] Add UnsavedDatasourceStore and DatasourceTestConnection…

[adrianSepiol]

…Button for testing datasource connections

Related pull requests (in order):

1. [ENHANCEMENT] support unsaved url generation for datasource api perses#4006
2. [BUGFIX] set correct ContentLength for request body perses#4007
3. This one
4. [ENHANCEMENT] integrate UnsavedDatasourceProvider in DatasourceDrawer perses#4008
5. [FEATURE] add test connection button to prometheus plugin plugins#620

Related issue:
perses/perses#1542

Description

Adds component for "test connection" button. It relays on UnsavedDatasourceStore to decouple the component from config that only app should be aware of. Wraps DashboardApp with provider so this button can be used for editing datasources on dashboard level. Provider for global and project datasources is added in perses/perses#4008

Screenshots

In perses/plugins#620

Checklist

• Pull request has a descriptive title and context useful to a reviewer.
• Pull request title follows the [<catalog_entry>] <commit message> naming convention using one of the
  following catalog_entry values: FEATURE, ENHANCEMENT, BUGFIX, BREAKINGCHANGE, DOC,IGNORE.
• All commits have DCO signoffs.

UI Changes

• Changes that impact the UI include screenshots and/or screencasts of the relevant changes.
• Code follows the UI guidelines.
• E2E tests are stable and unlikely to be flaky.
  See e2e docs for more details. Common issues include:
  • Is the data inconsistent? You need to mock API requests.
  • Does the time change? You need to use consistent time values or mock time utilities.
  • Does it have loading states? You need to wait for loading to complete.