Draft: Calculate max/min for DECIMAL columns

[paultiplady]

Problem

Targets must know the width of columns in order to be able to
correctly represent them. For example, BigQuery distinguishes
DECIMAL and BIGDECIMAL types, and a SQL database like target-mysql
or target-postgres would need to be able to reconstruct the actual
precision/scale values in order to be able to create a valid schema.

Proposed changes

To encode the DECIMAL column as JSONSchema, use Python Decimal
machinery to calculate the Maximum/Minimum values of a decimal
with the specified precision.

Types of changes

What types of changes does your code introduce to PipelineWise?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Since this changes the output schema, I think it's probably a breaking change. Some targets might not be able to handle a modification to the schema.

Checklist

• Description above provides context of the change
• I have added tests that prove my fix is effective or that my feature works
• Unit tests for changes (not needed for documentation changes)
• CI checks pass with my changes
• Bumping version in setup.py is an individual PR and not mixed with feature or bugfix PRs
• Commit message/PR title starts with [AP-NNNN] (if applicable. AP-NNNN = JIRA ID)
• Branch name starts with AP-NNN (if applicable. AP-NNN = JIRA ID)
• Commits follow "How to write a good git commit message"
• Relevant documentation is updated including usage instructions

[paultiplady]

This isn't complete, but I wanted to share this WIP and get your thoughts on whether it's an approach you'd be happy to upstream.

[paultiplady]

I think this should probably be configurable; by default the old behavior can be kept, but if you opt in to schema_includes_decimal_max_min config then this would be conditionally enabled.

Without a config flag, it'll be backwards-incompatible and will break existing target-schemas.