Remove hardcoded `payment_provider` options from `/places/search`

[dadofsambonzuki]

Currently, only square is accepted as a valid value for

e.g. square-test should be possible:

https://api.btcmap.org/v4/places/search/?payment_provider=square-test

Others such as coinos and blink should also be possible and so we should discuss allowing any value to be queried so we are not overly restrictive.

[bubelov]

Currently, only square is accepted as a valid value for

No, there is a whitelist, and it supported coinos from inception, which is also well documented. Blink is not cooperating in any way, so no point in including them. Search is our internal API and it won't support anyone except the sponsors

That "hardcoded" filter can't be simply removed because it would enable SQL injection, this argument is being inserted in a raw query

Adding square-test to a whitelist is the only thing we can do, if there is a need for that