fix/signing-config-setup

[arthurpalves]

What does this PR do

• Removes the runtime error when signing configuration is nil and, instead, maintains the project's signing settings as is (i.e.: in case of automatic signing)
• Adds/adjusts signing configuration when switching variants, not only during setup.
• When adding/adjusting signing configuration also include CODE_SIGN_STYLE and CODE_SIGN_IDENTITY
• Makes updateSigningConfig an optional step.
• Removes the necessity to have a matchURL parameter in the signing configuration, thus enabling locally installed provisioning profiles and certs

How can it be tested

1. Setup a new project using a variants.yml without a signing configuration. This should no longer fail. Instead, it should setup everything but maintain the existing signing config in Xcode.
2. Add a signing configuration to your variants.yml (either global or variant's specific) and switch variants. This should now modify the project's signing config in Xcode.

Task

resolves #214
resolves #215

Checklist:

• I ran make validation locally with success
• I have not introduced new bugs
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented on my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new errors