Introduce staff-only inline completion provider (#21739)

Release Notes:

- N/A

---------

Co-authored-by: Thorsten Ball <mrnugget@gmail.com>
Co-authored-by: Bennet <bennet@zed.dev>
Co-authored-by: Thorsten <thorsten@zed.dev>

Cargo.toml

@@ -141,6 +141,7 @@ members = [
     "crates/worktree",
     "crates/zed",
     "crates/zed_actions",
+    "crates/zeta",
 
     #
     # Extensions
@@ -325,6 +326,7 @@ workspace = { path = "crates/workspace" }
 worktree = { path = "crates/worktree" }
 zed = { path = "crates/zed" }
 zed_actions = { path = "crates/zed_actions" }
+zeta = { path = "crates/zeta" }
 
 #
 # External crates