agent: Improve the UX around interacting with MCP servers (#32622)

Still a work in progress! Todos before merging:

- [x] Allow to delete (not just turn off) an MCP server from the panel's
settings view
- [x] Also uninstall the extension upon deleting the server (check if
the extension just provides MCPs)
- [x] Resolve repository URL again
- [x] Add a button to open the configuration modal from the panel's
settings view
- [x] Improve modal UX to install and configure a non-extension MCP

Release Notes:

- N/A

---------

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>
Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>

crates/icons/src/icons.rs

@@ -263,6 +263,8 @@ pub enum IconName {
     ZedAssistantFilled,
     ZedBurnMode,
     ZedBurnModeOn,
+    ZedMcpCustom,
+    ZedMcpExtension,
     ZedPredict,
     ZedPredictDisabled,
     ZedPredictDown,