fix(stackable-webhook): Move certificate generation to the blocking thread

[NickLarsenNZ]

Note

Easier to review with whitespace hidden: https://github.com/stackabletech/operator-rs/pull/815/files?diff=split&w=1

Description

As per https://docs.rs/tokio/latest/tokio/task/fn.spawn_blocking.html,

In general, issuing a blocking call or performing a lot of compute in a future without yielding is problematic, as it may prevent the executor from driving other futures forward. This function runs the provided closure on a thread dedicated to blocking operations. See the CPU-bound tasks and blocking code section for more information.

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# Reviewer
- [ ] Code contains useful comments
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[Techassi]

LGTM, just one minor suggestion.