Currently, adding a cluster feature requires editing several files and
repeating the new feature name several times. This series reduces
the boilerplate to a single line (for non-experimental features), and
perhaps three for experimental features.
Closes#10488
* github.com:scylladb/scylla:
gms: feature_service: remove variable/helper function duplication
gms: feature: make `operator bool` implicit
gms: feature_service: remove feature variable duplication in enable()
gms: feature_service: remove feature variable declaration/definition duplication
gms: features: de-quadruplicate active feature names
gms: features: de-quadruplicate deprecated feature names
gms: feature_service: avoid duplicating feature names when listing known features
eeec502aee