[stableswap]: Implement maxNumAssets and maxTokensPerAsset to ensure multi-asset pools can't overflow

[AlpinYukseloglu]

Background

Even with extremely generous precision bounds, since we're dealing with non-linear k values for our stableswap CFMM it will almost always be possible to trigger an overflow if we don't bound the number of assets and the number of tokens for each asset. We should implement maxNumAssets and maxTokensPerAsset and add related checks to all pool creation and swapping logic to ensure that CFMM inputs are bounded.

Suggested Design

• Hard code maxNumAssets and maxTokensPerAsset to numbers that fit within our expected precision bounds. The specific numbers are TBD depending on the max number of assets we want to be able to accommodate in a single multi-asset pool

Acceptance Criteria

• All existing tests pass
• Error catching tests for out of bound inputs are added and pass