Proposal: Add "extendWorker" extension point

[garrettjstevens]

I'll explain the reasoning behind this extension point, and I'm certainly open to other ideas of ways to solve it if needed.

In Apollo, data about assemblies, features, sequences, etc. are stored in the session, and the Apollo display doesn't use web workers since it directly observes that store. For things like the regular JBrowse sequence track, though, it does use a web worker, and so can't read from the Apollo store in the session. We've gotten around this by creating a "FromConfigSequenceAdapter" when opening local GFF3s, but that duplicates a lot of data.

This extension point allows us to extend workers, which I've then used in Apollo to add an extra event listener in the worker. This allows the worker to query the main thread for data, which goes outside the normal RPC architecture.

If we ever do a server-side RPC driver, this extension point could also potentially be used to add web socket connections to the server.

[garrettjstevens]

Note from meeting: try using the same pattern the status callback uses in libRpc to add this communication channel.

[garrettjstevens]

This has been updated so that the extension point is evaluated during the "call" step of the base RPC driver. This enables the same behavior that is used in the WebWorkerRpcDriver where worker.on and worker.off can be accessed and set during the RPC call.

[codecov]

Codecov Report

Merging #3923 (56934be) into main (daf93bb) will increase coverage by 0.03%.
Report is 10 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #3923      +/-   ##
==========================================
+ Coverage   64.35%   64.38%   +0.03%     
==========================================
  Files        1004     1004              
  Lines       29862    29863       +1     
  Branches     7160     7160              
==========================================
+ Hits        19217    19227      +10     
+ Misses      10482    10471      -11     
- Partials      163      165       +2     

Files Changed	Coverage Δ
packages/core/rpc/BaseRpcDriver.ts	83.33% <100.00%> (+0.16%)	⬆️

... and 2 files with indirect coverage changes

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[cmdcolin]

regarding the need for the web worker to request data from the main thread, it is just a little bit outside the way we've done things so far so it's hard to evaluate from an architecture standpoint, but technically it is probably fine. an alternative architecture could involve the main thread to "sending serialized data to the worker via an rpc call", and then the worker deals with it, but if it seems like this particular architecture is better, we can go ahead with this.

[cmdcolin]

i'll just go ahead with merger as it is fine codewise for the extension point