Add percentile_cont and percentile_disc macros

[tripleaceme]

Summary

• Adds cross-database percentile_cont and percentile_disc macros with adapter.dispatch support
• Default implementation uses WITHIN GROUP (ORDER BY ...) syntax (Postgres, Redshift, Snowflake, Databricks)
• BigQuery override uses navigation function syntax: percentile_cont(column, value) OVER()
• Includes integration tests with seed data and assert_equal checks

Usage

-- Continuous percentile (e.g. median)
select {{ dbt_utils.percentile_cont('amount', 0.5) }} as median_amount
from orders

-- Discrete percentile
select {{ dbt_utils.percentile_disc('score', 0.75) }} as p75_score
from results

Test plan

• Integration tests pass on Postgres
• Integration tests pass on BigQuery
• Integration tests pass on Snowflake
• Integration tests pass on Redshift
• Integration tests pass on Databricks

Closes #1053