MapStaticAssets at runtime

### Is your feature request related to a problem? Please describe.
MapStaticAssets is a great set of reasonable defaults and very attractive as a feature. However, should you wish to use the functionality at run-time to build a generic static file server, the logic is pretty well out of reach which is a shame as the functionality is the most reasonable defaults I've seen in a builder.

### Describe the solution you'd like
Refactor the MsBuildTargets to a set of SDK capabilities that can with be called at build time or at runtime via builder WebHost.

### Additional context
It may be useful to add API surface area for new content during a process, such as if a user added a new HTML page in a CMS use-case. This would ensure all content could be treated uniformly.