Add function variantType

[Avogar]

Changelog category (leave one):

• New Feature

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Add function variantType that returns Enum with variant type name for each row.

Documentation entry for user-facing changes

Example:

CREATE TABLE test (v Variant(UInt64, String, Array(UInt64))) ENGINE = Memory;
INSERT INTO test VALUES (NULL), (42), ('Hello, World!'), ([1, 2, 3]);
SELECT variantType(v) from test;

┌─variantType(v)─┐
│ None           │
│ UInt64         │
│ String         │
│ Array(UInt64)  │
└────────────────┘

SELECT toTypeName(variantType(v)) FROM test LIMIT 1;

┌─toTypeName(variantType(v))──────────────────────────────────────────┐
│ Enum8('None' = -1, 'Array(UInt64)' = 0, 'String' = 1, 'UInt64' = 2) │
└─────────────────────────────────────────────────────────────────────┘

• Documentation is written (mandatory for new features)

Information about CI checks: https://clickhouse.com/docs/en/development/continuous-integration/

[robot-ch-test-poll3]

This is an automated comment for commit bd3bb05 with description of existing statuses. It's updated for the latest CI running

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[UnamedRus]

┌─toTypeName(variantType(v))──────────────────────────────────────────┐
│ Enum8('None' = -1, 'Array(UInt64)' = 0, 'String' = 1, 'UInt64' = 2) │
└─────────────────────────────────────────────────────────────────────┘

It is unlikely, to have single variant type with more than 127-128 types in it?
Ie as theoretical JSON type in it's realization, will contain more than 1 Dynamic(basically Variant) types(one per each key?)

[Avogar]

It is unlikely, to have single variant type with more than 127-128 types in it?

We support maximum 255 Variants, in Enum all variants with indexex 127+ will be negative (128 -> -128, 129 -> -127, ..., 255 (NULL index) -> -1).
In JSON/Dynamic if we will have more than 255 variants, we will cast the rarest into single String variant