feat: Add percentile_cont aggregate function

Adds exact percentile_cont aggregate function as the counterpart
to the existing approx_percentile_cont function.

This implementation:
- Calculates exact percentiles by storing all values in memory
- Supports WITHIN GROUP (ORDER BY ...) syntax
- Uses linear interpolation between values for continuous percentiles
- Handles all numeric types (integers, floats, decimals)
- Supports DISTINCT mode
- Includes GroupsAccumulator for efficient grouped aggregation
- Comprehensive test coverage

Closes #6714

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: adriangb

datafusion/functions-aggregate/src/lib.rs

@@ -81,6 +81,7 @@ pub mod hyperloglog;
 pub mod median;
 pub mod min_max;
 pub mod nth_value;
+pub mod percentile_cont;
 pub mod regr;
 pub mod stddev;
 pub mod string_agg;
@@ -123,6 +124,7 @@ pub mod expr_fn {
     pub use super::min_max::max;
     pub use super::min_max::min;
     pub use super::nth_value::nth_value;
+    pub use super::percentile_cont::percentile_cont;
     pub use super::regr::regr_avgx;
     pub use super::regr::regr_avgy;
     pub use super::regr::regr_count;
@@ -171,6 +173,7 @@ pub fn all_default_aggregate_functions() -> Vec<Arc<AggregateUDF>> {
         approx_distinct::approx_distinct_udaf(),
         approx_percentile_cont_udaf(),
         approx_percentile_cont_with_weight_udaf(),
+        percentile_cont::percentile_cont_udaf(),
         string_agg::string_agg_udaf(),
         bit_and_or_xor::bit_and_udaf(),
         bit_and_or_xor::bit_or_udaf(),