Implement straight-through signing for Service Accounts

[alexmatson-da]

Requirements:

• Add a dApp API middleware to accept API keys as authentication tokens
• If the user has authenticated through an API Key, the dApp API should not require approval for signing requests
• For external signing providers that require human approval, the SA should kick-off an in-memory asynchronous polling loop

We should be wary of keeping the HTTP request open for too long -- because the polling could take an arbitrary amount of time (until a human approves), then it is best to return some ID that the automation could poll from the client-side until the tx is executed