Make pluginManager param to getFetcher optional

[garrettjstevens]

getFetcher is supposed to be an alternative to openLocation for things like adapters that need to e.g. call a REST API instead of reading a file. This makes the pluginManager param for getFetcher optional so that its function signature and logic matches openLocation.

I found this while trying to use getFetcher in an adapter that retrieves sequences from the new Apollo server.

[codecov]

Codecov Report

Merging #3183 (3848bed) into main (c8c79c4) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #3183      +/-   ##
==========================================
- Coverage   59.49%   59.49%   -0.01%     
==========================================
  Files         671      671              
  Lines       28644    28645       +1     
  Branches     6934     6935       +1     
==========================================
  Hits        17043    17043              
- Misses      11279    11280       +1     
  Partials      322      322              

Impacted Files	Coverage Δ
packages/core/util/io/index.ts	51.78% <0.00%> (-0.95%)	⬇️
products/jbrowse-web/src/util.ts	48.33% <0.00%> (ø)

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[cmdcolin]

sounds good to me!