Generate sequence diagrams on request

[EttyKitty]

Checklist

• This is a new feature, not a refactoring suggestion
• I have checked for this feature in the Sourcery documentation
• I have checked there are no open requests referencing the same feature

Description

Right now, there is only a binary switch to control generation of sequence diagrams: you can either enable or disable them for all PR reviews.
I, personally, disabled them, as they take a lot of space and are not very useful for me to have in every PR.

What I want to suggest, is to add a "command" similar to $sourcery-ai review comment, that generates reviews on demand, something like $sourcery-ai diagram, that'll make the bot to respond and execute the default diagram generating behavior.
This way, you can disable diagram generation by default for all PRs and ask to generate them case by case, when you actually need them.

[Hellebore]

Hi - thanks for adding this. We were just discussing adding a feature like this today - it is definitely on our roadmap.