ShellInputProvider not usable outside its package

[mhayen]

Hi,

When i was trying the new test ShellInputProvider, there was sadly a compile error as its Builder was not public.
The example here only works when your in the org.springframework.shell.test package
Example

Created a very small PR to fix it.

[fmbenhassine]

Thank you for raising this and for contributing a PR. I planned the fix for the upcoming release 4.0.3.

[fmbenhassine]

Resolved with #1341