feat: add generic singleton Instance pattern for CustomItem and CustomRole

[MS-crew]

Description

Describe the changes
Adds CustomItem, CustomRole and typed wrappers for all subclasses (CustomWeapon, CustomKeycard, CustomArmor, CustomGrenade, CustomGoggles). Instead of fetching instances via Get(id) or Get(name) which are fragile since ids and names can change at any time with configs, other plugins can now reference a custom item or role directly via example MyCustomGoggles.Instance or MyCustomRole.Instance, getting full strongly typed access to all properties and methods without any casting. This makes cross plugin integrations simpler, less error prone, and more accessible to developers who aren't deeply familiar with the codebase.

What is the current behavior? (You can also link to an open issue here)

What is the new behavior? (if this is a feature change)

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
No

Other information:
Example usage

on another plugin via referance or another class

Types of changes

• 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 existing functionality to change)
• Documentations

Submission checklist

• I have checked the project can be compiled
• I have tested my changes and it worked as expected

Patches (if there are any changes related to Harmony patches)

• I have checked no IL patching errors in the console

Other

• Still requires more testing

[Saskyc]

I love this change it may introduce few issues with reflection, but it's just addition between the item being done and the item above.

It makes easier getting custom items and in between not removing the functionality to get by Id.

[Someone-193]

Please add the generic constraint ", new()" onto all the other ones. It means that all the inheritors of the class have to have a parameterless constructor, which is necessary for CI / CR

[MS-crew]

Please add the generic constraint ", new()" onto all the other ones. It means that all the inheritors of the class have to have a parameterless constructor, which is necessary for CI / CR

done @Someone-193

[Someone-193]

Looks good to go I guess. If you want another suggestion, maybe make Instance not nullable, but if you call the getter while it's null, the item auto-registers. Idk though