Fix #947 Enable to use app.client with passed token for single workspace apps

[seratch]

Summary

Refer to #947 for details. If we need to have some discussion about this, let's use the issue, not this PR.

For the comments about the changes in this PR, please feel free to join as a reviewer!

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #948 (03bbe21) into main (6007b1b) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #948   +/-   ##
=======================================
  Coverage   66.19%   66.19%           
=======================================
  Files          13       13           
  Lines        1207     1207           
  Branches      356      356           
=======================================
  Hits          799      799           
  Misses        338      338           
  Partials       70       70           

Impacted Files	Coverage Δ
src/App.ts	82.91% <100.00%> (ø)

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[stevengill]

LGTM