Refactor assume_schema_exists and expose it in aql.transform

[tatiana]

This is a follow-up for #1922. In that PR we allowed users to skip schema check & creation for aql.load_file, but we missed the fact that aql.transform and aql.transform_file had the same issue. This PR aims to address this limitation.

Changes included in this PR:

• Rename config load_table_schema_exists to assume_schema_exists
• Rename (load_file) argument schema_exists to assume_schema_exists
• Refactor where the check for assume_schema_exists happens. Before, it happened only inside the load_file_to_table. Now, it is part of create_schema_if_applicable. This makes this feature available in the aql.transform task as well
• Rename Database.create_schema_if_needed to Database.create_schema_if_applicable
• Expose assume_schema_exists in aql.transform
• Release 1.7.0a2

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +7.48 🎉

Comparison is base (55b7a8b) 84.81% compared to head (ca57b34) 92.29%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1925      +/-   ##
==========================================
+ Coverage   84.81%   92.29%   +7.48%     
==========================================
  Files         104       72      -32     
  Lines        5959     4246    -1713     
  Branches      678      512     -166     
==========================================
- Hits         5054     3919    -1135     
+ Misses        762      235     -527     
+ Partials      143       92      -51     

Flag	Coverage Δ
PythonSDK	92.29% <100.00%> (-0.07%)	⬇️
UTO	?

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Impacted Files	Coverage Δ
python-sdk/src/astro/__init__.py	100.00% <100.00%> (ø)
python-sdk/src/astro/databases/base.py	91.83% <100.00%> (-0.04%)	⬇️
python-sdk/src/astro/databases/databricks/delta.py	85.71% <100.00%> (ø)
python-sdk/src/astro/databases/postgres.py	96.73% <100.00%> (ø)
python-sdk/src/astro/settings.py	100.00% <100.00%> (ø)
python-sdk/src/astro/sql/operators/load_file.py	98.21% <100.00%> (ø)
python-sdk/src/astro/sql/operators/transform.py	90.62% <100.00%> (+0.62%)	⬆️

... and 2 files with indirect coverage changes

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