[stableswap]: Add max pool asset check to ValidateBasic()

[AlpinYukseloglu]

Background

Even with scaling factors and increased decimal precision, we still need to have a cap on the number of assets we can support in multi-asset pools.

This number should be somewhere between 4 and 8 assets per multi-asset pool and can be enforced by adding checks at pool creation. The simplest check we can add is in ValidateBasic(), although we should enforce this at lower levels as well as a guardrail.

Suggested Design

• Expand the input asset length check in ValidateBasic() to include a maximum (currently only checks that there are min 2 assets)
• Add any other relevant checks to CreatePool etc.

Acceptance Criteria

• All new and existing tests pass

[hieuvubk]

@AlpinYukseloglu As I read in the description, we don't allow pool with 3 assets right?

[AlpinYukseloglu]

I don't see why we wouldn't? Anything between 2 and the cap we set is fair game, although I think we should hold off on this until we finish bounding the rest of stableswap inputs and come to a conclusion on a reasonable maxNumAssets

[hieuvubk]

Alright I got a little misunderstanding here. We don't have a conclusion about the specific cap yet, right?

[AlpinYukseloglu]

Yes we haven't finalized it yet! It will come down to a number of factors tied to current open issues/PRs (not a major decision just one I'd prefer not to spend too much time reverting/changing)