Make raft-wal default when resource-apis is active

[dhiaayachi]

Description

This change the raft backend configuration behaviour as follow:

• If no backend is specified, experiments=["resource-apis"] is set and no boltdb raft file is on disk the backend will be WAL
• If WAL is set and no boltdb raft file is on disk WAL is used (this is haven't changed)
• If Boltdb is set boltDB will be used (this is haven't changed)
• If a db exist on disk using boltdb it will be used (this is haven't changed)

This is still missing the doc part that need to be added, I'm not sure if we add it right away or need to wait for the release.

Testing & Reproduction steps

Added config tests and manually tested that all the combinations above work as expected.

PR Checklist

• updated test coverage
• external facing docs updated

[ishustava]

🪵 🎉

[dhiaayachi]

@ishustava @rboyer I will merge this as is, we need to document this when we add the documentation for experiments=["resource-apis"]