Include initialOptions in BuildContext

Expose an `initialOptions` like property on the build context. This can make it easier to identify the context, and/or allow scenarios such as adding entrypoints from build A as watchFiles to build A where B manages the lifecycle of A (i.e. where A is a dependent of B, allow B to introspect A)

Start at the BuildContext API and trace how build options are represented and exposed. Determine the intended shape and lifecycle of an initialOptions-like property, then verify that it supports identifying a context and the dependent-build watchFiles scenario described in the issue.

Written by the indexing model from the issue text.