MergeTree & SipHash checksum big-endian support

[ltrk2]

The changes are meant to fix the 00961_checksums_in_system_parts_columns_table functional test suite.

Marks are now always serialized as little-endian to ensure interoperability between platforms. Composite structures are now transformed correctly from big-endian to little-endian when computing hashes using SipHash.

The handling of 128-bit checksums used for total checksum calculation consisting of two 64-bit unsigned integers was inconsistent in some cases: in big-endian mode CityHash returns them in a lo, hi component order and only the values of the components are big-endian, whereas SipHash returns a more canonical version as hi, lo with both the order of components and their values following big-endian convention. However, when reusing the output data structure of CityHash (i.e. std::pair<uint64_t, uint64_t>) SipHash now has a get128LoHi function that follows the lo, hi component order no matter what endianness the platform is so that the resulting hashes are comparable without having to know which algorithm they were produced by.

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[robot-clickhouse-ci-1]

This is an automated comment for commit 14977e6 with description of existing statuses. It's updated for the latest CI running
The full report is available here
The overall status of the commit is 🔴 failure

Check name	Description	Status
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SQLancer	Fuzzing tests that detect logical bugs with SQLancer tool	🟢 success
Sqllogic	Run clickhouse on the sqllogic test set against sqlite and checks that all statements are passed	🟢 success
Stateful tests	Runs stateful functional tests for ClickHouse binaries built in various configurations -- release, debug, with sanitizers, etc	🟢 success
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[rschu1ze]

As there are currently many failing tests in ClickHouse's CI, a policy was put in place which prevents merging of PRs with red tests, even if unrelated.
@ltrk2 as a contributor, you are of course not obliged to fix these tests (it's appreciated, though). Until the situation stabilizes, this PR is on hold. Maybe merge from master once in a while (please not too often), and with a bit of luck all tests are green.

[ltrk2]

@rschu1ze thanks for the heads-up!