Support custom entitlements path for update_code_signing_settings function

[dasheck0]

Resolves #91. Adds an option to provide entitlements_file_path to support custom setups (i.e. where each build target has a custom entitlements file).

Done:

• Update documentation to show how to use it
• Add parameter to action.yml and parse it as environment for fastlane to use
• Add it to fastlane update_code_signing_settings function

[dasheck0]

Closing this, since it is the wrong branche. Sorry!