Add injections field to Actor and Message

[AstraLuma]

This implements #389: provides a way for middleware to add keyword arguments that won't get serialized.

The way I set this up, caller arguments override message injections, which override actor injections.

This is deliberately meant to be a very minimal addition.

[AstraLuma]

@Bogdanp if these additions are acceptable, I'll happily write the documentation to go with.

[AstraLuma]

Any interest in reviewing this?

[Bogdanp]

Yes, but I don't have time right now and probably won't for a while. Sorry!

[AstraLuma]

I just updated to 1.12.0