Draft: Allow specification of bilby_pipe injection parameters

What?

This MR allows asimov to specify parameters for injections using bilby_pipe.

At present the implementation is very much a draft, as this is the first situation where we are attempting to specify injections using asimov, and as such the names of various parameters may need to change before we complete the implementation.

Why?

Previously it was not possible to specify injections using bilby_pipe.

Who?

This MR was prepared by @daniel-williams.