Path 1: update rcgen to version 0.12.1

[haurog]

As described in #5590 this PR shows all the necessary changes for update path 1.
In this solution, only minimal changes are necessary to achieve the goal described in #5590, but libp2p now depends on 2 different versions of rcgen.

Notes & open questions

Change checklist

I did not do the changes described below. I can do this once it is decided which update Path is the one to go.

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[dariusc93]

Thanks for the PR. I've always been curious on if libp2p would be able to build on risc-v, but never attempted myself so I am glad there is some interest on getting it working there.

Seeing you have another PR that is a "part 2" to this one, would it make more sense to have it all done in one PR?

[haurog]

Good to hear that other people are interested in getting it to compile on risc-v as well. The two PRs are two different ways to achieve the goal. I named them 'Path 1' and 'Path 2'. I hope the description in #5590 helps in understanding what and why I did it. The main reason for it was that I do not know the code base well enough to judge which approach is more feasible, so I submitted both versions for more experienced people to judge and improve upon.

[jxs]

replied on #5592 (review)