Review, Refactor, and Improve Token Swap implementation

An old token swap implementation was migrated from an internal codebase to the public sdk for broader consumption in pull request https://github.com/GalaChain/sdk/pull/468.

The scope of this issue is to take the migrated code, finish updating it to use more modern methods provided by the v2.x SDK, review it for potential security and refactoring improvements, and look for opportunities to share code with the recently drafted Token Sale feature (related issue: https://github.com/GalaChain/sdk/issues/443).

Open feedback left on the merged pull request https://github.com/GalaChain/sdk/pull/468 should be addressed in addition to the above higher level goals:

- fix, resolve, or delete old `todo` comments
- standardize use of `UserAlias`, `await resolveUserAlias()`, `UserAlias` in swap function params
- error handling updates - this code was written long before `ChainErrors` were standardized and provided in the SDK and there are several places where errors can be handling differently