External Data Sources: RPC implementation for the describe methods + ydb scheme describe support

[jepett0]

Implement describe external data sources / external tables RPCs. The implementations of the RPCs are almost identical and require deduplication (probably, via a common base class for both RPC implementation classes). However, we are in a hurry and this task:

• external data sources: deduplicate describe RPCs' code #14508

will be done separately.

ydb scheme describe is also half-baked (only the --format proto-json-base64 works as intended). Pretty format will be done in the next PRs in the series.

Note: YDB CLI is released separately and more often than YDB server versions. Users might end up with a newer CLI that supports enhanced external data source and table descriptions, but the server does not implement the RPCs yet. This users would get an error when they attempt to call the NYdb::NTable::TSession::DescribeExternalDataSource or DescribeExternalTable methods. This is a minor inconvenience, but not a blocker in my opinion. Only the targeted calls like ydb scheme describe my_external_data_source will fail (side note: ydb scheme ls -l -R does not use the new RPCs), so the existing user scripts should be just fine.

Changelog entry

Changelog category

• Not for changelog (changelog entry is not required)

Additional information

• external data sources: describe #14436

[pnv1]

CLI part LGTM

[Gazizonoki]

SDK LGTM

[github-actions]

⚪ 2025-02-13 15:23:11 UTC Pre-commit check linux-x86_64-release-asan for a0aa27b has started.
⚪ 2025-02-13 15:23:45 UTC Artifacts will be uploaded here
⚪ 2025-02-13 15:27:19 UTC ya make is running...
🟡 2025-02-13 16:54:25 UTC Some tests failed, follow the links below. This fail is not in blocking policy yet Going to retry failed tests...

Test history | Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
13932	13862	0	24	9	37

⚪ 2025-02-13 16:56:25 UTC ya make is running... (failed tests rerun, try 2)
🟡 2025-02-13 17:09:07 UTC Some tests failed, follow the links below. This fail is not in blocking policy yet Going to retry failed tests...

Test history | Ya make output | Test bloat | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
113 (only retried tests)	74	0	4	3	32

⚪ 2025-02-13 17:09:17 UTC ya make is running... (failed tests rerun, try 3)
🟡 2025-02-13 17:21:37 UTC Some tests failed, follow the links below. This fail is not in blocking policy yet

Test history | Ya make output | Test bloat | Test bloat | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
71 (only retried tests)	34	0	4	3	30

🟢 2025-02-13 17:21:44 UTC Build successful.
🟡 2025-02-13 17:22:16 UTC ydbd size 3.6 GiB changed* by +1.9 MiB, which is >= 100.0 KiB vs main: Warning

ydbd size dash	main: 54f9570	merge: a0aa27b	diff	diff %
ydbd size	3 879 350 288 Bytes	3 881 366 560 Bytes	+1.9 MiB	+0.052%
ydbd stripped size	1 358 503 552 Bytes	1 359 208 928 Bytes	+688.8 KiB	+0.052%

^{*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]

⚪ 2025-02-13 15:23:21 UTC Pre-commit check linux-x86_64-relwithdebinfo for a0aa27b has started.
⚪ 2025-02-13 15:23:55 UTC Artifacts will be uploaded here
⚪ 2025-02-13 15:27:35 UTC ya make is running...
🟡 2025-02-13 16:41:35 UTC Some tests failed, follow the links below. Going to retry failed tests...

Test history | Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
28102	25499	0	3	2464	136

⚪ 2025-02-13 16:44:37 UTC ya make is running... (failed tests rerun, try 2)
🟢 2025-02-13 17:02:38 UTC Tests successful.

Test history | Ya make output | Test bloat | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
216 (only retried tests)	88	0	0	1	127

🟢 2025-02-13 17:02:45 UTC Build successful.
🟡 2025-02-13 17:03:43 UTC ydbd size 2.1 GiB changed* by +1.1 MiB, which is >= 100.0 KiB vs main: Warning

ydbd size dash	main: 54f9570	merge: a0aa27b	diff	diff %
ydbd size	2 230 621 472 Bytes	2 231 806 376 Bytes	+1.1 MiB	+0.053%
ydbd stripped size	473 503 224 Bytes	473 719 096 Bytes	+210.8 KiB	+0.046%

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