[KQP RBO] Add avg aggregation function

[denis0x0D]

Changelog entry

#27661

Changelog category

• Not for changelog (changelog entry is not required)

Description for reviewers

...

[github-actions]

🟢 2025-11-24 07:18:33 UTC The validation of the Pull Request description is successful.

[github-actions]

⚪ 2025-11-24 07:21:03 UTC Pre-commit check linux-x86_64-relwithdebinfo for ef03194 has started.
⚪ 2025-11-24 07:21:21 UTC Artifacts will be uploaded here
⚪ 2025-11-24 07:23:28 UTC ya make is running...
🟡 2025-11-24 09:27:30 UTC Some tests failed, follow the links below. Going to retry failed tests...

Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
39534	36653	0	3	2837	41

⚪ 2025-11-24 09:27:42 UTC ya make is running... (failed tests rerun, try 2)
🟢 2025-11-24 09:39:14 UTC Tests successful.

Ya make output | Test bloat | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
78 (only retried tests)	62	0	0	0	16

🟢 2025-11-24 09:39:21 UTC Build successful.
🟡 2025-11-24 09:39:41 UTC ydbd size 2.3 GiB changed* by +171.2 KiB, which is >= 100.0 KiB vs main: Warning

ydbd size dash	main: 5230310	merge: ef03194	diff	diff %
ydbd size	2 455 477 888 Bytes	2 455 653 168 Bytes	+171.2 KiB	+0.007%
ydbd stripped size	523 286 616 Bytes	523 320 728 Bytes	+33.3 KiB	+0.007%

^{*please be aware that the difference is based on comparing your commit and the last completed build from the post-commit, check comparation}

[github-actions]

⚪ 2025-11-24 07:21:41 UTC Pre-commit check linux-x86_64-release-asan for ef03194 has started.
⚪ 2025-11-24 07:21:58 UTC Artifacts will be uploaded here
⚪ 2025-11-24 07:24:04 UTC ya make is running...
🟡 2025-11-24 08:37:22 UTC Some tests failed, follow the links below. This fail is not in blocking policy yet

Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
12576	12478	0	80	7	11

🟢 2025-11-24 08:37:34 UTC Build successful.
🟢 2025-11-24 08:38:08 UTC ydbd size 3.8 GiB changed* by +14.9 KiB, which is < 100.0 KiB vs main: OK

ydbd size dash	main: 8bf4163	merge: ef03194	diff	diff %
ydbd size	4 110 674 264 Bytes	4 110 689 568 Bytes	+14.9 KiB	+0.000%
ydbd stripped size	1 527 863 856 Bytes	1 527 875 536 Bytes	+11.4 KiB	+0.001%

^{*please be aware that the difference is based on comparing your commit and the last completed build from the post-commit, check comparation}