Eliminate Path(__file__) to locate resource files

[asnare]

Changes

What does this PR do?

This PR eliminates the use of Path(__file__) to locate file-based resources. In general:

• In the main code, this is buggy: modern Python should use importlib.resources instead.
• In tests, it's brittle and difficult to read because it's normally based on relative paths and traversal of many parent directories. It's also unnecessary, because pytest provides a mechanism for locating the project root. (Tests can also use importlib.resources, and probably should, but that's a bigger refactoring: this is a first step.)

Some incidental changes that are introduced:

• Type hints in a few places.
• Functional tests against Snowflake, Oracle and Presto have been consolidated into a single module. (The Snowflake functional tests have been moved into test_snowflake.py: test_databricks.py was misleading.)
• The PipelineConfig (and Step) classes have been made immutable. (An unnecessary __post__init() method has also been eliminated.)

Tests

• existing unit tests
• existing integration tests

[github-actions]

✅ 51/51 passed, 5 flaky, 4m34s total

Flaky tests:

• 🤪 test_transpiles_informatica_to_sparksql_non_interactive[False] (21.823s)
• 🤪 test_transpiles_informatica_to_sparksql (25.393s)
• 🤪 test_transpile_teradata_sql_non_interactive[False] (23.27s)
• 🤪 test_transpile_teradata_sql_non_interactive[True] (22.4s)
• 🤪 test_transpile_teradata_sql (7.249s)

_{Running from acceptance #3293}

[codecov]

Codecov Report

❌ Patch coverage is 37.50000% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.11%. Comparing base (b6a901f) to head (a9faad1).

Files with missing lines	Patch %	Lines
.../labs/lakebridge/assessments/dashboards/execute.py	0.00%	13 Missing ⚠️
.../labs/lakebridge/assessments/profiler_validator.py	0.00%	3 Missing ⚠️
...databricks/labs/lakebridge/assessments/profiler.py	0.00%	2 Missing ⚠️
...cks/labs/lakebridge/assessments/profiler_config.py	80.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2210   +/-   ##
=======================================
  Coverage   64.11%   64.11%           
=======================================
  Files         101      101           
  Lines        8675     8676    +1     
  Branches      902      898    -4     
=======================================
+ Hits         5562     5563    +1     
  Misses       2941     2941           
  Partials      172      172           

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[asnare]

I don't think this worked prior to this PR (or after): this is a pure refactoring.

But it does raise an interesting point: how is this tested?

@sundarshankar89: How can I check this?