Add optional params to repo config for Snowflake private key authentication

[ArijeetC]

Is your feature request related to a problem? Please describe.
When using private key authentication for Snowflake, the private key path and passphrase params must be defined in a config file and then the config file path must be added in the feature_store.yaml file (#2482 (comment)).

Instead of creating a separate config INI file just for these 2 params, they could be simply added in the feature_store.yaml file along with the other params.

Describe the solution you'd like
Adding optional params private_key and private_key_passphrase to the Snowflake config classes (SnowflakeRegistryConfig, SnowflakeOnlineStoreConfig, SnowflakeOfflineStoreConfig, SnowflakeMaterializationEngineConfig) so that they can be used in feature_store.yaml file directly.

Describe alternatives you've considered
None