fix(snowflake): Allow encrypted_extra field to be imported

[askldjd]

SUMMARY

The encrypted_extra field holds the keypair to authenticate to Snowflake. However, this field can't be imported currently.

See #22348.

TESTING INSTRUCTIONS

Here are my test steps:

1. Add a new Snowflake connection called "Snowflake UI" using the UI. Choose SQLAchemy as the option and use provide the keypair in the SECURE EXTRA following the instruction.
2. Construct a new snowflake-import.yaml in this format:

databases:
  - database_name: Snowflake from Import
    sqlalchemy_uri: $url
    cache_timeout: null
    expose_in_sqllab: true
    allow_run_async: false
    allow_ctas: false
    allow_cvas: false
    allow_csv_upload: false
    version: 1.0.0
    encrypted_extra: $encrypted_extra

where $url is the sqlachemy URL and encrypted_extra is the JSON encoded JSON block. So it looks something like this:

"{\n  \"auth_method\": \"keypair\",\n  \"auth_params\": {\n      \"privatekey_body\": \"-----BEGIN ENCRYPTED PRIVATE KEY-----\\nblablabla\\n-----END ENCRYPTED PRIVATE KEY-----\",\n      \"privatekey_pass\": \"mypassword\"\n  }\n}"

3. Run superset import_datasources -p snowflake-import.yaml
4. Inspect that dbs table and verify that both the UI entry and the imported entry are identical.

• Of course, test the connection from the UI too. :)

Prior to the fix, step 4 would fail. The encrypted_extra column would be empty for the imported entry.

ADDITIONAL INFORMATION

Fixes #22348

[github-actions]

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[betodealmeida]

This is great, @askldjd!

[codecov]

Codecov Report

Merging #22357 (b6b3145) into master (ff1d29c) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #22357      +/-   ##
==========================================
- Coverage   66.85%   66.85%   -0.01%     
==========================================
  Files        1847     1847              
  Lines       70560    70560              
  Branches     7737     7737              
==========================================
- Hits        47173    47171       -2     
- Misses      21380    21382       +2     
  Partials     2007     2007              

Flag	Coverage Δ
hive	52.53% <100.00%> (ø)
mysql	77.95% <100.00%> (ø)
postgres	78.02% <100.00%> (ø)
presto	52.42% <100.00%> (ø)
python	81.23% <100.00%> (-0.01%)	⬇️
sqlite	76.48% <100.00%> (ø)
unit	50.91% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/models/core.py	90.50% <100.00%> (ø)
superset/db_engine_specs/bigquery.py	81.68% <0.00%> (-1.00%)	⬇️

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