Implement schema uri insertion for Snowflake

[villebro]

SQL Lab queries weren't passing the selected schema into Snowflake URI, forcing the user to either define the schema in the connection string or prefixed before the table name.

[codecov-io]

Codecov Report

Merging #5474 into master will decrease coverage by 0.01%.
The diff coverage is 25%.

@@            Coverage Diff            @@
##           master   #5474      +/-   ##
=========================================
- Coverage   59.11%   59.1%   -0.02%     
=========================================
  Files         372     372              
  Lines       23751   23759       +8     
  Branches     2758    2758              
=========================================
+ Hits        14041   14043       +2     
- Misses       9695    9701       +6     
  Partials       15      15

Impacted Files	Coverage Δ
superset/db_engine_specs.py	52.9% <25%> (-0.36%)	⬇️

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[villebro]

@mistercrunch any chance to get this merged? Would help in testing #5487

[villebro]

Logic moved to #5474 .