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.lock

@@ -2619,6 +2619,7 @@ dependencies = [
  "clock",
  "cocoa 0.26.0",
  "collections",
+ "credentials_provider",
  "feature_flags",
  "futures 0.3.31",
  "gpui",
@@ -3477,6 +3478,19 @@ dependencies = [
  "crc",
 ]
 
+[[package]]
+name = "credentials_provider"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+ "futures 0.3.31",
+ "gpui",
+ "paths",
+ "release_channel",
+ "serde",
+ "serde_json",
+]
+
 [[package]]
 name = "criterion"
 version = "0.5.1"
@@ -6985,6 +6999,7 @@ dependencies = [
  "client",
  "collections",
  "copilot",
+ "credentials_provider",
  "deepseek",
  "editor",
  "feature_flags",
@@ -16682,6 +16697,7 @@ dependencies = [
  "command_palette_hooks",
  "component_preview",
  "copilot",
+ "credentials_provider",
  "db",
  "diagnostics",
  "editor",