Standardise the validators and others simliar implementations.

[TheSCREWEDSoftware]

I only can talk for Validators for SKUs, currently any new implementation requires (manually) to add a piece of code as validador, which is a bad pratice. We should have a function or class for that purpose.

What does this mainaly fixes?

• New implementations may (my case) forget to manually create a validator for that implementation.
  Example: SKU requires the character selection to be added to the cart, should be mandatory, currently each implementation has that validation either copy pasted or new validator to achieve the same.

• Reduces code size and code repetition.
  Having the same code or similiar code in each implementation to do the same, could be their own function or class.

• Allows a easier choice of validators for developers or new implementation.
  Example:

• AccountSelector | You required to be logged in

• CharacterSelector | You required to be logged in and too choose a character prior to add to a cart.

• GuildSelector | You required to be logged in, choose a character and choose a guild.
  and other validators.

Having each Validator having the required Validators to it could be good or bad (for nesting).

GuildSelector requires CharacterSelector which requires AccountSelector as example

| AccountSelector
| - CharacterSelector
| - - GuildSelector

Edit: IsGiftable or smth also.