Cleaning up an if helper for fixed length command
Cleaning up an if helper for fixed length command

- Adding if_command_fixed_length and removing if_command_is_fixed_length with a cleaner implementation to do if else on fixed length commands
- Deprecating if_command_is_fixed_length
- JIRA: ZAPP-621

project-chip/zap,project-chip/zap,project-chip/zap,project-chip/zap