WIP: Plugin API v1

[sk1p]

Nothing finished yet, this PR is just a reminder of what "threads" I'm currently working on.

@jhgee this is the PR to follow if you want to keep up to date on the plugin work

Fixes #992

TODO

• Rebase after Context.run_udfs improvements #1011 is merged

Contributor Checklist:

• I have added or updated my entry in the creators.json file
• I have added a changelog entry for my contribution
• I have added/updated documentation for all user-facing changes
• I have added/updated test cases
• I have included the rebuilt production build of the client (only if changes were made to the GUI)

Reviewer Checklist:

• /azp run libertem.libertem-data passed

[codecov]

Codecov Report

Merging #1013 (9e54af1) into master (08a53ec) will decrease coverage by 8.20%.
The diff coverage is 41.41%.

@@            Coverage Diff             @@
##           master    #1013      +/-   ##
==========================================
- Coverage   69.27%   61.06%   -8.21%     
==========================================
  Files         268      273       +5     
  Lines       12572    12663      +91     
  Branches     1769     1776       +7     
==========================================
- Hits         8709     7733     -976     
- Misses       3521     4634    +1113     
+ Partials      342      296      -46     

Impacted Files	Coverage Δ
src/libertem/plugins/v1/discover.py	25.00% <25.00%> (ø)
src/libertem/web/messages.py	88.40% <33.33%> (-2.51%)	⬇️
src/libertem/web/plugins.py	35.48% <35.48%> (ø)
src/libertem/plugins/v1/fields.py	38.88% <38.88%> (ø)
src/libertem/plugins/v1/base.py	40.90% <40.90%> (ø)
src/libertem/common/buffers.py	82.83% <100.00%> (+0.05%)	⬆️
src/libertem/plugins/v1/__init__.py	100.00% <100.00%> (ø)
src/libertem/udf/base.py	93.72% <100.00%> (+0.02%)	⬆️
src/libertem/web/server.py	47.77% <100.00%> (+1.11%)	⬆️
src/libertem/io/dataset/dm.py	27.41% <0.00%> (-62.10%)	⬇️
... and 35 more

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