Add support for Hashicorp Vault to store Tessera keys

[chris-j-h]

Add Hashicorp Vault to supported key vaults that can be used with Tessera. Features include:

• Store generated public/private key pairs in a Vault configured with a version 2 k/v secret engine
• Retrieve public/private key pairs and previous versions of key pair from a pre-configured Vault
• TLS communication between Tessera and the Vault server is supported. Non-TLS communication is also allowed for development/test purposes
• Vault config can be provided in configfile, and the corresponding CLI options have been added to provide this information for key generation
• GetSecretData and SetSecretData added as arguments for the KeyVaultService methods. This allows for the same method signatures to be used in the Hashicorp and Azure implementations despite each provider's different interpretation of what constitutes a secret

[chris-j-h]

Changes have been made to support v2 of the k/v secrets engine (i.e. versioning of secrets and retrieval of previous versions of a secret)