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First, index type "vector_similarity" is more speaking and user-friendly
than "usearch". Second, we should not expose the name of the library
doing the job (usearch). Of course, the docs will continue to mention
usearch (credit where credit is due).

Existing setting `allow_experimental_usearch_index` was marked obsolete.
A new settings `allow_experimental_vector_similarity_index` was added.

Index types 'annoy' and 'usearch' were removed and replaced by
'vector_similarity' indexes in an earlier commit.

This means unfortuantely, that if customers have tables with these
indexes and upgrade, their database might not start anymore - the
system loads the metadata at startup, thinks something is wrong with
such tables, and halts immediately.

This commit adds support for loading and attaching such indexes back.
Data insert or use (search) return an error which recommends a migration
to 'vector_similarity' indexes. The implementation is generally similar
to what has recently been implemented for 'full_text' indexes [1, 2].

[1] ClickHouse#64656
[2] ClickHouse#64846

USearch (similar to FAISS) allows to specify the distance function,
quantization, and various HNSW meta-parameters for index creation and
sarch. Some users wished for greater configurability, so let's expose
them.

Index creation now requires either
- 2 parameters (with the other 4 parameters taking on default values), or
- 6 parameters for full control

This commit also remove quantization `f64` (that would be upsampling).

Previously, only this syntax to create a skip index worked:

   INDEX index_name column_name TYPE vector_similarity('hnsw', 'L2Distance')

Now, this syntax will work as well:

  INDEX index_name column_name TYPE vector_similarity(hnsw, L2Distance)