Fix schema sampling apoc meta data

[androna-xm]

Description

This PR exposes the APOC sample parameter used in schema discovery so that users can control sampling when calling get_structured_schema and get_schema.

• Add an optional sample parameter to get_structured_schema and get_schema.
• Thread the sample value through to the apoc.meta.data calls (via {sample: $SAMPLE}).
• Keep current behaviour by default (sample=1000).
• Allow sample=-1 to disable sampling and perform a full scan.

This addresses the improvement requested in #350.

Type of Change

• [x ] New feature
• Bug fix
• Breaking change
• Documentation update
• Project configuration change

Complexity

Complexity: Low

Small, localized change in schema.py that adds a parameter and passes it through to existing APOC procedures without altering the rest of the API surface.

How Has This Been Tested?

• Unit tests
• E2E tests
• [ x] Manual tests

Checklist

The following requirements should have been met (depending on the changes in the branch):

• [x ] Documentation has been updated
• Unit tests have been updated
• E2E tests have been updated
• Examples have been updated
• New files have copyright header
• [ x] CLA (https://neo4j.com/developer/cla/) has been signed
• [x ] CHANGELOG.md updated if appropriate