feat: add validate command to validate configuration files

[samcm]

Adds a validate command, which just checks that the provided config is valid and exits 0 if so.

go run main.go validate --config /tmp/test-config-bad-task.yaml

ERRO[0000] configuration validation failed               error="configuration validation failed:\n  - test[0] 'test-1': task[0] 'sleep': error parsing task config for sleep: time: invalid duration \"invalid-duration\""
exit status 1