insights: [FE] Implement dashboard page API methods

Created by: vovakulikov

At the moment dashboard page uses (explicitly and implicitly) these API (data) methods

• getDashboards- get all dashboards info to be able to load dashboard insights further, display dashboard list in the select component
• getDashboardInsights - to show insight within dashboard.

We need to replace these methods API calls with methods from a single API context that we implement in this issue. https://github.com/sourcegraph/sourcegraph/issues/25874