Handles logic relating to the authentication of a Vault client

Defines the Vault HTTP client and constructors

Defines the mock Vault client

Core protocols for interacting with the Vault API.

Code to resolve secrets from a map of environment configuration. This is
useful for service startup configs using the app-id authentication scheme.

If a client is not provided, the code initializes one from the `VAULT_URL`,
`VAULT_APP_ID`, and `VAULT_USER_ID` environment variables.

Storage logic for Vault secrets and their associated leases.

Interface for communicating with a Vault key value version 1 secret store (generic)

Interface for communicating with a Vault key value version 2 secret store (kv)