feat: do not check for context existence if a custom context file is set

[lburgazzoli]

As today, it a custom context file is set, then the code check for the existence of the context file and fails if it does not exist, however, this is not true for custom configuration location. This PR aims to make config and context behaving consistently.

Verification Steps

1. Set RHOASCONFIG to an not existing file
2. Run rhoas
3. No error should be reported and the RHOASCONFIG file should have been created

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation change
• [ ] Other (please specify)

[rkpattnaik780]

The change has been released in v0.51.2.