Async code in ProducesTickets implementation

[tvsfx]

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Is your feature request related to a problem? Please describe.
I am writing a custom implementation of the ProducesTickets trait. As part of encryption and decryption, I occasionally need to make some asynchronous database calls (when I need to rotate my STEK), which is currently not possible because the encrypt/decrypt methods are synchronous.

Describe the solution you'd like
I was wondering if there are any plans to support this use case, or whether there's an existing solution for this that I'm missing.

Describe alternatives you've considered
As a workaround, I am thinking of spinning up a separate task to monitor the keys and rotate when necessary (to avoid calling block_on in a new runtime). I am not sure whether this is the best way to go about this issue.

[cpu]

I was wondering if there are any plans to support this use case, or whether there's an existing solution for this that I'm missing.

I think #850 is the most relevant issue for capturing plans in this area. There's also the start of some work in #1648, but only the client-side and so nothing in relation to ProducesTickets yet.

[djc]

Yeah, I think a potential workaround is to have some kind of channel to a background task.