Refactor MPT/DEX integration to use std::visit style as opposed to if/else when accessing issue in Asset.

[gregtatcam]

Summary

Replace if/else with std::visit when accessing Issue/MPTIssue in Asset

Motivation

Style consistency and efficiency

Solution

Introduce Asset::visit() helper function

[vlntb]

https://github.com/gregtatcam/rippled/pull/56/files