[5.6] Add Builder::whereJsonContains()

[staudenmeir]

At the moment we can query JSON columns for exact matches:

DB::table('users')->where('options->language', 'en')->get();

This PR adds whereJsonContains() with support for MySQL and PostgreSQL:

// Example value in the `options` column: {"languages":["de","en","fr"]}
DB::table('users')->whereJsonContains('options->languages', 'en')->get();
DB::table('users')->whereJsonContains('options->languages', ['en', 'de'])->get();
DB::table('users')->whereJsonContains('options->languages', DB::raw("'\"en\"'"))->get();

The method expects a PHP value and converts it to JSON. Raw expressions are also supported.

The PostgresGrammar replaces the column name's last -> with a ->> . This is required for where(), but has to be undone for whereJsonContains().

Resolves laravel/ideas#296.

[sisve]

This PR adds whereJsonContains() with support for MySQL and PostgreSQL:

What happens with other database engines? Do we get a NotSupportedException or anything to help the end-user track down why their queries fail? (Or do they work and return something weird?)

[staudenmeir]

The queries just fail. I added an exception.

[taylorotwell]

Asserting the SQL query matches what you expect seems pointless to test. Test that it actually does something useful / correct.

[staudenmeir]

@taylorotwell Do you mean integration tests? Do we support these for MySQL/PostgreSQL?