Chore: Handle pool creation w/ only two pools

[fabianschu]

Status Quo

• right now it is possible to create an IndexPool with only two pools from the start
• apparently what happens internally, is that it creates a pool with the TWO_TOKEN specialization
• for this specialization it is not possible to register further tokens with the vault after creation (TOKENS_LENGTH_MUST_BE_2), therefore it is a problem for the index pool

Acceptance Criteria

• check if there is a straightforward way to use a general pool specialization also for two-token pools
• if not, a validation has been added to make sure that new pools start w/ at least three tokens

[Zitzak]

As long as IndexPool inherits from BaseWeightPool it is not possible to call PoolSpecialization.GENERAL. So we decided on adding a require to make sure the pool gets created with more then 2 tokens.

[Zitzak]

After our recent call with Index, it came up that we will look into ways of creating 2 token pools after the main issues all done. That's why I'm opening this issue again