Implementation of simple lightweight event base class

[rtuck99]

I was going to use this as part of implementing blSampleStatus updates however in the end have plumped for a slightly different solution*, however I think this may be of use for other things.

Therefore I present this as a draft PR for general comment/feedback and if people want to use it.

*which turned out not to work, so I may still use it

[codecov]

Codecov Report

Attention: Patch coverage is 96.77419% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.11%. Comparing base (fa31d79) to head (1ff6b78).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #606      +/-   ##
==========================================
+ Coverage   80.04%   80.11%   +0.07%     
==========================================
  Files          97       98       +1     
  Lines        6814     6845      +31     
==========================================
+ Hits         5454     5484      +30     
- Misses       1360     1361       +1     

Components	Coverage Δ
i24 SSX	60.20% <ø> (ø)
hyperion	96.77% <96.77%> (+<0.01%)	⬆️
other	94.95% <ø> (ø)

[DominicOram]

I think this really needs discussion with the wider bluesky community about why this isn't a core feature of bluesky already. I'm concerned that by doing this we're diverging more from standard bluesky things.