Implement lazy loading for offline and online stores #2566

The offline and online stores are eagerly loaded in FeatureStore, leading unnecessary dependencies to be pulled in. For example, as #2560 reported, the AWS Lambda feature server does not work with a Snowflake offline store, even though the offline store is not strictly required for serving online features.

The offline and online stores should be lazily loaded instead. This will allow the Snowflake dependencies to removed from the AWS Lambda feature server Dockerfile. Note that #2560 was fixed by #2565, but this issue tracks a longer-term solution.