assistant: Add a Configuration page (#15490)

- [x] bug: setting a key doesn't update anything
- [x] show high-level text on configuration page to explain what it is
- [x] show "everything okay!" status when credentials are set
- [x] maybe: add "verify" button to check credentials
- [x] open configuration page when opening panel for first time and
nothing is configured
- [x] BUG: need to fix empty assistant panel if provider is `zed.dev`
but not logged in


Co-Authored-By: Thorsten <thorsten@zed.dev>

Release Notes:

- N/A

---------

Co-authored-by: Thorsten <thorsten@zed.dev>
Co-authored-by: Nate Butler <iamnbutler@gmail.com>
Co-authored-by: Thorsten Ball <mrnugget@gmail.com>

crates/assistant/Cargo.toml

@@ -47,6 +47,7 @@ indoc.workspace = true
 language.workspace = true
 language_model.workspace = true
 log.workspace = true
+markdown.workspace = true
 menu.workspace = true
 multi_buffer.workspace = true
 ollama = { workspace = true, features = ["schemars"] }