Add the `auth.EnvVars` function (#2403)

## Changes
This PR adds the auth.EnvVars function, which is a list of all
environment variables that the SDK uses to read auth configuration.

This is useful for spawning subprocesses since you can unset all auth
environment variables to clean up the environment before configuring the
auth.

It's used in #2278 today and will also be useful for the `databricks
bundle exec` command.
 
## Tests
Unit test.

libs/auth/env.go

@@ -38,3 +38,22 @@ func GetEnvFor(name string) (string, bool) {
 
 	return "", false
 }
+
+// EnvVars returns the list of environment variables that the SDK reads to configure
+// authentication.
+// This is useful for spawning subprocesses since you can unset all auth environment
+// variables to clean up the environment before configuring authentication for the
+// child process.
+func EnvVars() []string {
+	out := []string{}
+
+	for _, attr := range config.ConfigAttributes {
+		if len(attr.EnvVars) == 0 {
+			continue
+		}
+
+		out = append(out, attr.EnvVars...)
+	}
+
+	return out
+}

libs/auth/env_test.go

@@ -79,3 +79,51 @@ func TestGetEnvFor(t *testing.T) {
 	assert.False(t, ok)
 	assert.Empty(t, out)
 }
+
+func TestAuthEnvVars(t *testing.T) {
+	// Few common environment variables that we expect the SDK to support.
+	contains := []string{
+		// Generic attributes.
+		"DATABRICKS_HOST",
+		"DATABRICKS_CONFIG_PROFILE",
+		"DATABRICKS_AUTH_TYPE",
+		"DATABRICKS_METADATA_SERVICE_URL",
+		"DATABRICKS_CONFIG_FILE",
+
+		// OAuth specific attributes.
+		"DATABRICKS_CLIENT_ID",
+		"DATABRICKS_CLIENT_SECRET",
+		"DATABRICKS_CLI_PATH",
+
+		// Google specific attributes.
+		"DATABRICKS_GOOGLE_SERVICE_ACCOUNT",
+		"GOOGLE_CREDENTIALS",
+
+		// Personal access token specific attributes.
+		"DATABRICKS_TOKEN",
+
+		// Databricks password specific attributes.
+		"DATABRICKS_USERNAME",
+		"DATABRICKS_PASSWORD",
+
+		// Account authentication attributes.
+		"DATABRICKS_ACCOUNT_ID",
+
+		// Azure attributes
+		"DATABRICKS_AZURE_RESOURCE_ID",
+		"ARM_USE_MSI",
+		"ARM_CLIENT_SECRET",
+		"ARM_CLIENT_ID",
+		"ARM_TENANT_ID",
+		"ARM_ENVIRONMENT",
+
+		// Github attributes
+		"ACTIONS_ID_TOKEN_REQUEST_URL",
+		"ACTIONS_ID_TOKEN_REQUEST_TOKEN",
+	}
+
+	out := EnvVars()
+	for _, v := range contains {
+		assert.Contains(t, out, v)
+	}
+}