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`.

cmd/password_reader.go

@@ -16,7 +16,7 @@ type StdInPasswordReader struct {
 }
 
 // ReadPassword reads a password from stdin.
-func (_ StdInPasswordReader) ReadPassword() (string, error) {
+func (StdInPasswordReader) ReadPassword() (string, error) {
 	pwd, err := terminal.ReadPassword(int(os.Stdin.Fd()))
 	return string(pwd), err
 }