Add static credential support to RabbitMQ secret engine

[f4z3r]

I will add the API documentation and the general documentation to this PR in the coming days.

[f4z3r]

Thanks for the review @schavis !

[schavis]

Suggested change

	1. Alternatively, request credentials for a static role by reading from the
	Alternatively, request credentials for a static role by reading from the

Missed this one in the first review, apologies. No need to number steps if there's only one step :)

[austingebauer]

Hi, @f4z3r! This looks like a reasonable feature to add to the RabbitMQ secrets engine. However, we're starting to notice a pattern around how static role credential rotation is taking form across the various secrets engines (e.g., database, LDAP, AWS) that support it. The priority queue based rotation mechanism is being copy-pasted around. We want to take the time to look at how we can introduce rotation mechanisms to the various secrets engines without this copy-pasting. Ideally, it would be much easier for secrets engine developers to provide secrets rotation without having to maintain the mechanism that performs the rotation.

For that reason, we'd like to hold off on merging this. If it's important for your use case, I can help provide some information for how you could run this as an external plugin in your Vault deployment. Happy to hear your thoughts.

[f4z3r]

Hi @austingebauer , this definitely makes sense. Especially considering that there are quite a few little elements that one needs to consider that are not super obvious directly from the code when one looks at it (what happens in the event of a crash of Vault, how high can a rotation delay end up being, etc). It should be noted though, that as far as I remember the AWS static credentials do not use a priority queue. I tried to base the RMQ implementation off of that initially, but had some issues with rotations being massively delayed.

We are currently not yet relying on tihs for our use case, but might want to utilise such features in the future.

If you are open for some help on the refactoring, I would be happy to support.

[austingebauer]

Thanks for understanding, @f4z3r! I don't have specific details to share yet, but I appreciate you offering to help out.

I'm going to close this PR since it's not something we'll merge. It would be neat to revisit this once we have something available to hook into.