Introduce a platform_type enum (ForgePlatformType)

[MoralCode]

Description
This change introduces a new python Enum representing the "platform type" - a new word for the same concept we were already using, primarily in ContributorUUID. This concept assigns a magic integer of 1 for github contributors and 2 for gitlab. This enum both formalizes and renames this magic number into a ForgePlatformType, allowing us to keep the usefulness of knowing the type (i.e. API compatibility) for a given git forge, while moving it to a more appropriate structure. Previously these values were naievely stored in their own database table, platform which is being deprecated.

This PR contributes to one step of #293

Notes for Reviewers

Functionally this just adds an Enum that isnt being used anywhere.

Future PRs will address the primary task of connecting this Enum to the forge_instances table

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[github-actions]

[pylint] _{reported by reviewdog 🐶}
W0611: Unused ForgePlatformType imported from collectoss.util.enums (unused-import)