Get task git-access-token from Key Vault

**What is the problem you're trying to solve**
The lifecycle of a git-access-token configured in a task's source triggers may be different from that of the task. This means that sooner or later, that token may expire and need to be renewed, for instance. If a single token is used in multiple tasks, this forces updating each individual task, possibly days after the token has expired.

**Describe the solution you'd like**
It should be possible to retrieve this token from a Key Vault (like other secrets used in tasks), allowing for a single secret to be managed and taking advantage of the secret management features already available in Key Vaults.