Configurable paywall template instead of the default button

Currently we add a [default button to unlock the paywall](https://github.com/getAlby/lightning-publisher-wordpress/blob/master/includes/class-bln-publisher-paywall.php#L283-L292)

to be more flexible there we should allow people to render their own HTML template instead of the default button.
this gives great flexibility in designing how the paywall looks

We have all the JS and the REST endpoints to allow to do this.

Start in includes/class-bln-publisher-paywall.php around lines 283-292, then trace the existing JavaScript and REST endpoints mentioned in the issue. Determine how a caller can provide a custom HTML template in place of the default unlock button. Done means the paywall can render the supplied template while preserving the existing unlock behavior.

Written by the indexing model from the issue text.