Use Literals instead (str, Enum) in models

[sbasan]

Pull Request summary:

Use Literal over the Enums. Justification:

• Both provides static typing safety ant intellisense (type-hints).
• (str, Enum) can be problematic as __str__ != __format__ for Enum
• When instantiating pydantic Field defined as Literal no explicit import of specific Enum is needed

Support Literal when using as_default and as_global (additional optional argument must be provided).

additional minor refactoring:

• use serialization_alias and validation_alias when applicable
• replace RefId with Global[UUID]
• customize public items in catalystwan/models/configuration/feature_profile/sdwan/policy_object/__init__.py and catalystwan/models/policy/__init__.py using PEP-562 features

Description of changes:

[Add more in depth analysis of what changed, provide logs, examples of usage]

Checklist:

• Make sure to run pre-commit before committing changes
• Make sure all checks have passed
• PR description is clear and comprehensive
• Mentioned the issue that this PR solves (if applicable)
• Make sure you test the changes

[jpkrajewski]

lgtm