rcmgr: provide an easy API for getting the limits that are being enforced

[BigLep]

Situation/usecase

Kubo has commands to output for a user what the current resource manager/accountant limits are. According to @Jorropo, there isn't a clean/simple API for getting this back as a ConcreteLimitConfig. As a result, in Kubo, we are actually rebuilding at runtime the limit config we passed to go-libp2p (see discussion here: ipfs/kubo#9680 (comment) ). It would be ideal to have ground truth from go-libp2p on what it is actually using vs. what Kubo thinks it's using.