[5.6] Add Builder::whereJsonContains()

[staudenmeir]

This is a re-submit of #24258 to discuss your comment about better tests:

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

I assume that you mean integration tests? As far as I see, we don't support these for MySQL/PostgreSQL.
I agree that only testing the generated SQL isn't ideal, but I don't know how to improve it (of course, I tested the code with real databases).

For the future: Is a re-submit your preferred way to ask about a closed PR?

[taylorotwell]

Isn't the string concatenation happening in compileJsonContains opening up people to SQL injection? The user can inject any string value directly into that string concatenation operation.

[staudenmeir]

Only with raw expressions. Otherwise Grammar::whereJsonContains() uses a ? parameter.

[taylorotwell]

Thanks!