Add GitHub Copilot Chat Support (#14842)

# Summary

This commit implements Github Copilot Chat support within the existing
Assistant panel/framework. It required a little bit of trickery and
internal API modification, as Copilot doesn't use the same
authentication-style as all of the existing providers, opting to use
OAuth and a short lived API key instead of a straight API key. All
existing Assistant features should work.

Release Notes:
- Added Github Copilot Chat support
([#4673](https://github.com/zed-industries/zed/issues/4673)).

## Screenshots
<img width="1552" alt="A screenshot showing a conversation between a
user and Github Copilot Chat within the Zed editor."
src="https://github.com/user-attachments/assets/73eaf6a2-792b-4c40-a7fe-f763bd6417d7">

---------

Co-authored-by: Bennet Bo Fenner <bennet@zed.dev>

Cargo.lock

@@ -2749,6 +2749,7 @@ dependencies = [
  "async-compression",
  "async-std",
  "async-tar",
+ "chrono",
  "client",
  "clock",
  "collections",
@@ -2759,6 +2760,7 @@ dependencies = [
  "gpui",
  "http_client",
  "indoc",
+ "isahc",
  "language",
  "lsp",
  "menu",
@@ -2767,10 +2769,13 @@ dependencies = [
  "paths",
  "project",
  "rpc",
+ "schemars",
  "serde",
  "serde_json",
  "settings",
  "smol",
+ "strum",
+ "task",
  "theme",
  "ui",
  "util",
@@ -6020,6 +6025,7 @@ dependencies = [
  "anyhow",
  "client",
  "collections",
+ "copilot",
  "ctor",
  "editor",
  "env_logger",
@@ -6028,6 +6034,7 @@ dependencies = [
  "google_ai",
  "gpui",
  "http_client",
+ "inline_completion_button",
  "language",
  "log",
  "menu",