Create a builder-style object for WasiExecutionContext

[radu-matei]

It would very convenient to a builder object for the execution context -- this would allow importers to choose which host imports to use when instantiating a specific engine -- and it would make it much clearer what host imports are available.

Currently, core WASI, experimental HTTP, and WASI NN are imported by default -- the default should probably just be core WASI, with the other options (and as we add more) available through the builder.

The builder will probably have to contain references to the store and linker used for pre-instantiation.

This could also be a good exercise in thinking about allowing implementers to define their own host imports.