[FEATURE] Add support for `median(<value>)`

[aalva500-prog]

Is your feature request related to a problem?
Yes. PPL lacks SPL’s median function median(<field>).

median(<field>) and percentile50(<field>) are functionally equivalent - they both return the 50th percentile value of the specified field.

However, there are some subtle differences:

• median() is specifically designed for finding the middle value and may have slight performance optimizations for this exact use case.
• percentile50() is part of the more general percentile function family, which can calculate any percentile (1-99)

In practice, both will give you the same result for the 50th percentile calculation. The choice between them is often a matter of code readability and intent - median() makes it immediately clear you want the median value, while percentile50() shows you're working with percentile calculations.

What solution would you like?
Support this shortcut and rewrite it to perc50(<value>), which is already supported with pushdown optimization.

What alternatives have you considered?
N/A

Do you have any additional context?
N/A

[RyanL1997]

#4234 merged