Hashicorp Vault integration for transaction inputs

[brunotm]

Is your feature request related to a problem? Please describe.

Transaction inputs can only be defined at the inputs field, which makes it hard to have version controlled tests without having to strip sensitive information and even harder to have test credentials to be rotated.

Describe the solution you'd like

Inputs should have a special field like "vault://secret/path" that would specify a secret path from a Vault system and populate the inputs with the key/value pairs available within the secret path.

This would must happen at before parsing/decoding at each transaction execution so existing scheduled tests will always have the latest data available from vault.

Additional context

The implementation should allow for pluggable data sources, eg: etcd://data/path.