feat: allow developer to get supersetclient instance

[kristw]

🏆 Enhancements

In the existing implementation, SupersetClient.getInstance() will always throw error unless you also pass a SupersetClient instance as an argument. Then it will just return what you pass in as an argument back.

I believe the original intention was to force developer to call SupersetClient.configure() before calling .getInstance() but even after calling .configure(). The .getInstance() itself is still not useful by itself. Calling it without argument, hoping to obtained the configured SupersetClient results in an Error. (I want to get the host name from the already configured singleton) and if I already have the SupersetClient instance, I would not bother calling this function.

This PR modifies the code to preserve the original intention, but allow .getInstance() to return the configured SupersetClient if it already exists. (The only way for it to exist is somebody must have called .configure() before that.)

Technically this is breaking but I have checked the relevant repositories and none call .getInstance() (not surprising though given its limited usefulness, if any)

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[codecov]

Codecov Report

Merging #552 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #552   +/-   ##
=======================================
  Coverage   23.13%   23.13%           
=======================================
  Files         290      290           
  Lines        6783     6783           
  Branches      676      676           
=======================================
  Hits         1569     1569           
  Misses       5176     5176           
  Partials       38       38           

Impacted Files	Coverage Δ
...kages/superset-ui-connection/src/SupersetClient.ts	100.00% <100.00%> (ø)

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[williaster]

ah interesting. I think you're right about the original intention, in any case this makes more sense to me 👍