x/gamm: return tokens joined in pool-level JoinPoolNoSwap

[AlpinYukseloglu]

Background

Our current JoinPoolNoSwap implementation does not let the caller know how much of the tokens they passed in were successfully joined. We get around this in the message-level JoinPoolNoSwap by assuming the tokens passed in are all successfully joined, which in general is a fair assumption since we only pass in amounts that are calculated to be perfect joins. Still, it is a little hacky and it would likely be cleaner to have a direct thread from the assets that were successfully joined to the amount the function returns as joined. cc: @p0mvn

Suggested Design

• Add tokensJoined return value to pool-level JoinPoolNoSwap

Acceptance Criteria

• Existing JoinPoolNoSwap tests are updated to check the new return value and pass

[pysel]

hey @AlpinYukseloglu, wdyt about reopening #2685?