Add CredentialsProvider to silence keychain prompts in development (#25266)

This PR adds a new `CredentialsProvider` trait that abstracts over
interacting with the system keychain.

We had previously introduced a version of this scoped just to Zed auth
in https://github.com/zed-industries/zed/pull/11505.

However, after landing https://github.com/zed-industries/zed/pull/25123,
we now have a similar issue with the credentials for language model
providers that are also stored in the keychain (and thus also produce a
spam of popups when running a development build of Zed).

This PR takes the existing approach and makes it more generic, such that
we can use it everywhere that we need to read/store credentials in the
keychain.

There are still two credential provider implementations:

- `KeychainCredentialsProvider` will interact with the system keychain
(using the existing GPUI APIs)
- `DevelopmentCredentialsProvider` will use a local file on the file
system

We only use the `DevelopmentCredentialsProvider` when:

1. We are running a development build of Zed
2. The `ZED_DEVELOPMENT_AUTH` environment variable is set
- I am considering removing the need for this and making it the default,
but that will be explored in a follow-up PR.

Release Notes:

- N/A

Cargo.toml

@@ -16,6 +16,7 @@ members = [
     "crates/auto_update",
     "crates/auto_update_ui",
     "crates/breadcrumbs",
+    "crates/buffer_diff",
     "crates/call",
     "crates/channel",
     "crates/cli",
@@ -31,10 +32,10 @@ members = [
     "crates/context_server",
     "crates/context_server_settings",
     "crates/copilot",
+    "crates/credentials_provider",
     "crates/db",
     "crates/deepseek",
     "crates/diagnostics",
-    "crates/buffer_diff",
     "crates/docs_preprocessor",
     "crates/editor",
     "crates/evals",
@@ -233,6 +234,7 @@ component_preview = { path = "crates/component_preview" }
 context_server = { path = "crates/context_server" }
 context_server_settings = { path = "crates/context_server_settings" }
 copilot = { path = "crates/copilot" }
+credentials_provider = { path = "crates/credentials_provider" }
 db = { path = "crates/db" }
 deepseek = { path = "crates/deepseek" }
 diagnostics = { path = "crates/diagnostics" }