Introduction of a new Rucio account for managing Relval workflows

[anpicci]

Impact of the new feature
WMAgent, likely MSOutput

Is your feature request related to a problem? Please describe.
P&R wants to treat the relval workflows separately from the production ones in terms of data placement to improve the handling of such workflows on their side.
The main topic is introduced in #12069 , and this issue is intended to track down the work that needs to be done on the Rucio account side after addressing the main issue.

Describe the solution you'd like
Talking with @hassan11196 about this issue, there are two scenarios to deal with:

1. for usual relvals, we would need a new Rucio account, like wma_relval, in place of wma_prod, to be used in MSOutput to create the Rucio rules for such workflows (I think it should be eventually specified in the secrets of the relval agents.
2. for relvals needed to submit to prod nodes (aka special relvals), P&R would like to use the new Rucio account, wma_relval, instead of wma_prod. In my understanding, the Rucio account is fixed once and for all by the secrets configuration of the agent, and then if a relval workflow is injected in a prod node, it will inherit the related Rucio account, wma_prod. One naive solution would be to have a prod-relval agent, with only the Rucio account different from the usual prod agents, but I suppose this would become very easily a unsustainable solution. @hassan11196 was wondering if it would be possible to workaround the configs at the post-production level, forcing the agent to use wma_relval instead of wma_prod when the workflow is a relval one and still injected in a prod agent.

Describe alternatives you've considered
None at the moment, but a follow-up with @amaltaro and @haozturk is needed to better outline the solutions for this issue