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

crates/zed/src/main.rs

@@ -723,7 +723,7 @@ fn handle_open_request(request: OpenRequest, app_state: Arc<AppState>, cx: &mut
 
 async fn authenticate(client: Arc<Client>, cx: &AsyncApp) -> Result<()> {
     if stdout_is_a_pty() {
-        if *client::ZED_DEVELOPMENT_AUTH {
+        if *credentials_provider::ZED_DEVELOPMENT_AUTH {
             client.authenticate_and_connect(true, cx).await?;
         } else if client::IMPERSONATE_LOGIN.is_some() {
             client.authenticate_and_connect(false, cx).await?;