Prefer using literal types over enums

[vdusek]

• We should prefer typing.Literal types over enum.Enum.

Motivation

• Better developer experience - Literals can be passed directly as values, without requiring additional imports.
• Type safety is preserved - When used with a type checker, Literal types provide the same level of safety as enums.
• Fewer footguns - The usage of enums can easily lead to bugs like this one: fix: Fix force permissions serialization for Actor calls #575.
• Consistency across our tooling - We already prefer literals in Crawlee and the SDK.

Scope

• This primarily applies to constants currently defined as enums, especially those in: https://github.com/apify/apify-shared-python/blob/master/src/apify_shared/consts.py.

Compatibility

• This is a breaking change, so it should be tracked and implemented as part of apify-python-client v3.