Update internal repos-config file format

[rjbou]

At the moment, repos-config file list repositories with optional fields:
repositories: [ <string> <URL> { {<string>}+ } { <int> } ... ]. The last two optional arguments for each repository are the trust anchors (fingerprints of the trusted signing identities, and the quorum, i.e. how many of them are required for a signature to be accepted (see manual.
We need to add some information per repository (see #4933, #5966), that are optional too, and handling these list of optional elements is not the best, even if the file is not meant to be read by a human.

Proposal: use one section per repository, we would have

repo "default" {
  url: "https://opam.ocaml.org"
}
repo "local" {
  url: "git+file:///home/user/opam-repo"
  another-prop: ...
}
repo "another" {
  url: "git+https://..."
  quorum: 2
  fingerprints: "..."
}

Like that we have each repository have its own properties well defined.