feat(multiple-auth): improvements in multiple auth implementation

[sufyankhanrao]

What

This PR adds improvements in the implementation of multiple authentication schemes. The improvements include the addition of auth-validation exception type which is to be raised when auth validation fails and the early returning of flow in case of OR authentication group. The changes also contain some refactoring of the code.

Why

1. To fix a logical problem with the OR authentcation group
2. To add a custom exception for auth validation for enhancing the developer experience

closes #47

Type of change

Select multiple if applicable.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause a breaking change)
• Tests (adds or updates tests)
• Documentation (adds or updates documentation)
• Refactor (style improvements, performance improvements, code refactoring)
• Revert (reverts a commit)
• CI/Build (adds or updates a script, change in external dependencies)

Dependency Change

If a new dependency is being added, please ensure that it adheres to the following guideline https://github.com/apimatic/apimatic-codegen/wiki/Policy-of-adding-new-dependencies-in-the-core-libraries

Breaking change

If the PR is introducing a breaking change, please ensure that it adheres to the following guideline https://github.com/apimatic/apimatic-codegen/wiki/Guidelines-for-maintaining-core-libraries

Testing

List the steps that were taken to test the changes

Checklist

• My code follows the coding conventions
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added new unit tests

[asadali214]

LGTM