Validator client - Improve readability - NO FUNCTIONAL CHANGE (#13468)

* Improve `NewServiceRegistry` documentation.

* Improve `README.md`.

* Improve readability of `registerValidatorService`.

* Move `log` in `main.go`.

Since `log` is only used in `main.go`.

* Clean Tos.

* `DefaultDataDir`: Use `switch` instead of `if/elif`.

* `ReadPassword`: Remove unused receiver.

* `validator/main.go`: Clean.

* `WarnIfPlatformNotSupported`: Add Mac OSX ARM64.

* `runner.go`: Use idiomatic `err` handling.

* `waitForChainStart`: Avoid `chainStartResponse`mutation.

* `WaitForChainStart`: Reduce cognitive complexity.

* Logs: `powchain` ==> `execution`.

runtime/service_registry.go

@@ -31,7 +31,7 @@ type ServiceRegistry struct {
 	serviceTypes []reflect.Type           // keep an ordered slice of registered service types.
 }
 
-// NewServiceRegistry starts a registry instance for convenience
+// NewServiceRegistry starts a registry instance for convenience.
 func NewServiceRegistry() *ServiceRegistry {
 	return &ServiceRegistry{
 		services: make(map[reflect.Type]Service),