YDB-6486 Implement external blob precharge in read iterator

[SammyVimes]

Co-authored-by: Aleksei Borzenkov snaury@ydb.tech

[github-actions]

⚪ 2024-10-02 10:43:27 UTC Pre-commit check for 92f4e0e has started.
⚪ 2024-10-02 10:46:17 UTC Build linux-x86_64-release-asan is running...
🟢 2024-10-02 11:32:12 UTC Build successful.
⚪ 2024-10-02 11:32:29 UTC Tests are running...
🔴 2024-10-02 13:30:35 UTC Some tests failed, follow the links below.

Test history | Test log

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
11271	11194	0	10	35	32

🟢 2024-10-02 13:31:06 UTC ydbd size 5.5 GiB changed* by +17.1 KiB, which is < 100.0 KiB vs stream-nb-24-3: OK

ydbd size dash	stream-nb-24-3: 576812d	merge: 92f4e0e	diff	diff %
ydbd size	5 940 464 048 Bytes	5 940 481 520 Bytes	+17.1 KiB	+0.000%
ydbd stripped size	1 488 914 400 Bytes	1 488 922 720 Bytes	+8.1 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}

[github-actions]

⚪ 2024-10-02 10:43:28 UTC Pre-commit check for 92f4e0e has started.
⚪ 2024-10-02 10:46:15 UTC Build linux-x86_64-release-clang14 is running...
🟢 2024-10-02 11:22:42 UTC Build successful.

[github-actions]

⚪ 2024-10-02 10:44:11 UTC Pre-commit check for 92f4e0e has started.
⚪ 2024-10-02 10:46:59 UTC Build linux-x86_64-relwithdebinfo is running...
🟢 2024-10-02 11:45:30 UTC Build successful.
⚪ 2024-10-02 11:45:45 UTC Tests are running...
🔴 2024-10-02 13:29:54 UTC Some tests failed, follow the links below.

Test history | Test log

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
39811	34561	0	8	5212	30

🟢 2024-10-02 13:30:42 UTC ydbd size 8.2 GiB changed* by +48.5 KiB, which is < 100.0 KiB vs stream-nb-24-3: OK

ydbd size dash	stream-nb-24-3: 2fc19d5	merge: 92f4e0e	diff	diff %
ydbd size	8 765 070 992 Bytes	8 765 120 656 Bytes	+48.5 KiB	+0.001%
ydbd stripped size	479 260 520 Bytes	479 263 048 Bytes	+2.5 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}