Feature request : allow the smithy-build CLI to source models from some SPI-loaded interface

It'd be nice if smithy-build provided a way for services to be loaded programmatically. A usecase would be for it to allow models resulting from translations from other formats, like openapi, typespec, proto, etc.

At the moment, the only way to achieve this is by abusing the projection mechanism, which seems neither correct nor ergonomic. 
