with_options_service_account_key_file support added (#107)

README.md

@@ -37,29 +37,40 @@ To run an example with environment variables:
 PROJECT_ID=<your-google-project-id> cargo run --example crud
 ```
 
-## Firestore database client instance lifecycle
+## Firestore database client instance and lifecycle
 
 To create a new instance of Firestore client you need to provide at least a GCP project ID.
+It is not recommended creating a new client for each request, so it is recommended to create a client once and reuse it whenever possible.
+Cloning instances is much cheaper than creating a new one.
+
 The client is created using the `Firestore::new` method:
 ```rust
 use firestore::*;
 
 // Create an instance
 let db = FirestoreDb::new(&config_env_var("PROJECT_ID")?).await?;
 ```
+This is the recommended way to create a new instance of the client, since it 
+automatically detects the environment and uses credentials, service accounts, Workload Identity on GCP, etc. 
+Look at the section below [Google authentication](#google-authentication) for more details. 
 
-It is not recommended creating a new client for each request, so it is recommended to create a client once and reuse it whenever possible. 
-Cloning instances is much cheaper than creating a new one.
+In cases if you need to create a new instance explicitly specifying a key file, you can use:
+
+```rust
+FirestoreDb::with_options_service_account_key_file(
+        FirestoreDbOptions::new(config_env_var("PROJECT_ID")?.to_string()),
+        "/tmp/key.json".into()
+).await?
+```
 
-In case if it is needed you can also create a new client instance using preconfigured token source.
+or if you need even more flexibility you can use a preconfigured token source and scopes with:
 
-For example:
 ```rust
 FirestoreDb::with_options_token_source(
         FirestoreDbOptions::new(config_env_var("PROJECT_ID")?.to_string()),
         gcloud_sdk::GCP_DEFAULT_SCOPES.clone(),
         gcloud_sdk::TokenSourceType::File("/tmp/key.json".into())
-).await?;
+).await?
 ```
 
 ## Fluent API

src/db/mod.rs

@@ -113,6 +113,18 @@ impl FirestoreDb {
         .await
     }
 
+    pub async fn with_options_service_account_key_file(
+        options: FirestoreDbOptions,
+        service_account_key_path: std::path::PathBuf,
+    ) -> FirestoreResult<Self> {
+        Self::with_options_token_source(
+            options,
+            gcloud_sdk::GCP_DEFAULT_SCOPES.clone(),
+            gcloud_sdk::TokenSourceType::File(service_account_key_path),
+        )
+        .await
+    }
+
     pub async fn with_options_token_source(
         options: FirestoreDbOptions,
         token_scopes: Vec<String>,