`howl`: stream some sample environment

I would like howl to basically fake an entire run, which will include sample environment data.

for this to work, start with 1 PV with streaming a sine wave (basically 0-10-0 on a loop) and it'd be nice to test a few different datatypes for this

this also needs a placeholder for each PV in the run start with type f144 like this one for ev44: https://github.com/ISISComputingGroup/saluki/blob/main/src/howl.rs#L48 so that a downstream consumer such as a filewriter can write this information. Docs for this here: https://github.com/ess-dmsc/kafka-to-nexus/blob/main/documentation/commands.md#data-streams-alt-writer-modules