Add `spring.shell.enabled` property for global en/disable

[onobc]

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Maybe we should add spring.shell.enabled property as traditionally most of a boot autoconfigs have global flag for feature.

Originally posted by @jvalkeal in https://github.com/spring-projects/spring-shell/issues/374#issuecomment-1047521778

[ullenboom]

Is this still on the table?

[onobc]

Hi @ullenboom , we still want to do it but we have not had time to do it. We would happily accept a contribution. :)