feat: Environment variable interpolation in the Gman configuration file

src/providers/azure_key_vault.rs

@@ -1,3 +1,4 @@
+use crate::config::deserialize_optional_env_var;
 use crate::providers::SecretProvider;
 use anyhow::{Context, Result};
 use azure_identity::DefaultAzureCredential;
@@ -30,6 +31,7 @@ use validator::Validate;
 #[serde(deny_unknown_fields)]
 pub struct AzureKeyVaultProvider {
     #[validate(required)]
+    #[serde(default, deserialize_with = "deserialize_optional_env_var")]
     pub vault_name: Option<String>,
 }