Vault backend support to Secret Service

Secret service contains a pluggable security backend framework and currently, it was implemented to support database-backed storage [1]. However, this is not recommended for production usage as credentials are saved in plain text. Vault [2] is a very popular credential storage tool and it is better if we can have backend implementation to support credential storage in vault

[1] https://github.com/apache/airavata-mft/tree/master/services/secret-service/server/src/main/java/org/apache/airavata/mft/secret/server/backend
[2] https://www.vaultproject.io/