Faker: Override generator expression in select statement

[brybacki]

Description

Faker: Add a possibility to override the generator expression in the select statement. It is possible to do something like this:

SELECT name FROM person WHERE name='#{Name.first_name}' limit 1

Additional context and related issues

Release notes

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## Section
* Fix some things. ({issue}`issuenumber`)

[brybacki]

Using a condition in the where clause to specify the pattern to use for generation is an abuse of the semantics of the where clause.

Specifically, when a column is compared to a string, the result should be such that the values of that column match exactly the provided string. This is not the case here.

That is true, thanks for the comment. I think we can do a virtual column. Or maybe we can drop this PR and just improve documentation.

[brybacki]

There is already a way to do what we wanted from SQL with Faker connector function random_string.

SELECT id, random_string('#{Name.first_name}') AS name
FROM faker.default.table
LIMIT 50
UNION ALL
SELECT id random_string('#{Name.first_name} #{Name.first_name}') AS name
FROM faker.default.table
LIMIT 50