fix(gsheets): bug fix for private sheets

[AAfghahi]

SUMMARY

We weren't passing in impersonate_user as a payload in the front end so it was not allowing users to add private gsheets to datasets or to query them.

This also has a fix for a small 500 error that was occurring when editing a private gsheet. It was occurring because encrypted_extra is sometimes a string and sometimes a dict. I'm just running a simple check to make sure.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

Create a new private gsheet and add a service key.
Go to sql lab
Query it

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #17019 (ff5581c) into master (03cdbac) will decrease coverage by 0.00%.
The diff coverage is 50.00%.

❗ Current head ff5581c differs from pull request most recent head fc58b0d. Consider uploading reports for the commit fc58b0d to get more accurate results

@@            Coverage Diff             @@
##           master   #17019      +/-   ##
==========================================
- Coverage   76.72%   76.71%   -0.01%     
==========================================
  Files        1030     1030              
  Lines       55088    55090       +2     
  Branches     7480     7481       +1     
==========================================
  Hits        42264    42264              
- Misses      12573    12575       +2     
  Partials      251      251              

Flag	Coverage Δ
javascript	70.89% <0.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
...c/views/CRUD/data/database/DatabaseModal/index.tsx	43.57% <0.00%> (-0.23%)	⬇️
superset/db_engine_specs/gsheets.py	75.00% <100.00%> (ø)

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[betodealmeida]

@yousoph let's discuss user impersonation when we have the chance, and see if we should enable this by default for DBs that support it.

[rosemarie-chiu]

🏷 2021.40