backfill: merge vectors from a temp forward index into a vector index

Previously mutation of values in a vector index during backfill was
prohibited, effectively taking the table offline while the index is
built. This patch does a few things:

* Re-enables mutations on vector indexes undergoing backfill.
* Changes the temporary index created for vector indexes to capture
  changes during backfill from a VECTOR index to a FORWARD index.
* Changes the schema of the temporary index so that it is keyed by the
  index prefix and primary key columns, with the vector as a payload
  column.
* Refactors the mvcc merger to take a functor argument to merge
  individual entries, with the existing kv merger as the default.
* Adds a second merger that re-encodes vector index entries read from
  the temporary index.

Because forward indexes don't record values for tombstone entries, we
don't delete vectors from the vector index that are deleted during
backfill. Since reading from a vector index requires an index join on
the PK, this doesn't create a correctness issue, but it could cause
accuracy issues if a large number of vector index entries are deleted
during backfill.

Fixes: #144443
Release note (sql change): Tables with vector indexes will no longer be
taken offline while the vector index builds.

Author: mw5h

pkg/sql/backfill/BUILD.bazel

@@ -40,6 +40,7 @@ go_library(
         "//pkg/sql/sem/idxtype",
         "//pkg/sql/sem/transform",
         "//pkg/sql/sem/tree",
+        "//pkg/sql/span",
         "//pkg/sql/sqlerrors",
         "//pkg/sql/types",
         "//pkg/sql/vecindex",