✨ feat(credential): implement credential loading and conversion

- add struct `Credential` for deserializing Google API credentials
- implement `load_json_file` to load credentials from a JSON file
- implement `From<Credential> for yup_oauth2::ApplicationSecret` for conversion

src/credential.rs

@@ -0,0 +1,51 @@
+use std::{env, fs, path::PathBuf};
+
+use google_gmail1::yup_oauth2;
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct Installed {
+    pub(crate) client_id: String,
+    pub(crate) project_id: Option<String>,
+    pub(crate) auth_uri: String,
+    pub(crate) token_uri: String,
+    pub(crate) auth_provider_x509_cert_url: Option<String>,
+    pub(crate) client_secret: String,
+    pub(crate) redirect_uris: Vec<String>,
+}
+
+/// Struct for google credentials
+#[derive(Debug, Serialize, Deserialize)]
+pub struct Credential {
+    installed: Option<Installed>,
+}
+
+impl Credential {
+    /// Load the credential from a file.
+    pub fn load_json_file(path: &str) -> Self {
+        let home_dir = env::home_dir().unwrap();
+
+        let path = PathBuf::new().join(home_dir).join(".config").join(path);
+        let json_str = fs::read_to_string(path).expect("could not read path");
+
+        serde_json::from_str(&json_str).expect("could not convert to struct")
+    }
+}
+
+impl From<Credential> for yup_oauth2::ApplicationSecret {
+    fn from(value: Credential) -> Self {
+        let mut out = Self::default();
+        if let Some(installed) = value.installed {
+            out.client_id = installed.client_id;
+            out.client_secret = installed.client_secret;
+            out.token_uri = installed.token_uri;
+            out.auth_uri = installed.auth_uri;
+            out.redirect_uris = installed.redirect_uris;
+            out.project_id = installed.project_id;
+            // out.client_email = installed.client_email;
+            out.auth_provider_x509_cert_url = installed.auth_provider_x509_cert_url;
+            // out.client_x509_cert_url = installed.client_x509_cert_url;
+        }
+        out
+    }
+}