PS-11167: Add DISTANCE() for VECTOR data type

[catalinbp]

Implement SQL DISTANCE(vector, vector, metric) and the VECTOR_DISTANCE() synonym for vector similarity queries. Supported metrics: EUCLIDEAN (L2), MANHATTAN (L1), COSINE, and DOT.

Core library (vector-common/vector_distance.*):

• Runtime SIMD dispatch across Scalar, SSE4.2/NEON, AVX2, AVX-512F, and SVE2 tiers; per-kernel target attributes, no global -march=native
• Dim-aware wide/narrow dispatch (dims >= 16 use widest tier; smaller vectors use 128-bit tier to avoid AVX setup overhead)
• Unaligned load intrinsics throughout: VECTOR data may be misaligned (e.g. SUBSTR blobs); on modern CPUs unaligned and aligned loads have identical throughput when data is aligned; aligned loads would fault on misaligned inputs without a performance benefit. Loads that span a 64-byte cache-line boundary may still be slower (two line fetches); that depends on runtime address, not on loadu vs load, and is not avoided by switching to aligned intrinsics
• Float32 SIMD accumulation with double-precision horizontal sum and scalar tail: preserves correctness for large dimensions and extreme values (e.g. 2e38 Euclidean distance) without sacrificing SIMD width
• Manhattan (L1): sum of absolute differences;

[percona-mhansson]

Getting unit test failure on Mac:

[==========] Running 1 test from 1 test suite.
[----------] Global test environment set-up.
[----------] 1 test from VectorDistanceTest
[ RUN      ] VectorDistanceTest.UnalignedMatchesAligned
/home/martin/gitroot/worktrees/ps-11167/9.7/unittest/gunit/vector_distance-t.cc:208: Failure
Value of: is_aligned_to(ma, alignof(float))
  Actual: true
Expected: false

[  FAILED  ] VectorDistanceTest.UnalignedMatchesAligned (0 ms)
[----------] 1 test from VectorDistanceTest (0 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test suite ran. (0 ms total)
[  PASSED  ] 0 tests.
[  FAILED  ] 1 test, listed below:
[  FAILED  ] VectorDistanceTest.UnalignedMatchesAligned

 1 FAILED TEST

Which is weird, but clang+linux+arm seems to align stuff a bit weirdly

[catalinbp]

Seems like buf_a + 1 isn't guaranteed misaligned on apple silicon. I'll push a change that tries harder to miss align