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/credentials_provider/Cargo.toml

@@ -0,0 +1,21 @@
+[package]
+name = "credentials_provider"
+version = "0.1.0"
+edition.workspace = true
+publish.workspace = true
+license = "GPL-3.0-or-later"
+
+[lints]
+workspace = true
+
+[lib]
+path = "src/credentials_provider.rs"
+
+[dependencies]
+anyhow.workspace = true
+futures.workspace = true
+gpui.workspace = true
+paths.workspace = true
+release_channel.workspace = true
+serde.workspace = true
+serde_json.workspace = true