Add function repeat

[ucasfl]

I hereby agree to the terms of the CLA available at: https://yandex.ru/legal/cla/?lang=en

For changelog. Remove if this is non-significant change.

Category (leave one):

• New Feature

Short description (up to few sentences):

Add function repeat related to issue-6648

Detailed description (optional):

repeat function has two arguments, the first argument is String and the second is unsigned integer, both of them can be constant or nonconstant. So, we can use this function as follows:

SELECT repeat('abc', 10);
DROP TABLE IF EXISTS defaults;
CREATE TABLE defaults
(
        strings String,
        u64 UInt64
)ENGINE = Memory();

INSERT INTO defaults values ('abc', 56) ('sdfgg', 200) ('xywq', 4) ('plkf', 0);

SELECT repeat(strings, u64) FROM defaults;
SELECT repeat(strings, 10) FROM defaults;
SELECT repeat('abc', u64) FROM defaults;

[alexey-milovidov]

Now it's ready for merge!
I've made a few edits in #7048

[amosbird]

it's #7049 btw