Make it possible to put recurring payments against pledges

Add 'supportsRecurringForPledges to the payment processor classes. If TRUE, then allow backoffice user to enter recurring contribution for pledge payment(s) in the contribution form.

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We've been recording recurring contributions against pledges for all our pledge-using customers for years.

The BaseIPN class has support for this but we also need to add this patch to make it work

https://github.com/civicrm/civicrm-core/commit/e876b55e0f146ae291d624663aac469229805f2a

If we can't get this feature enabled in core an alternative is to use an assigned parameter to the template like $showRecurBlock - which would put the logic into the form as to whether it should be shown & then it would be easily overridable in an extension with no patch (or, horror of horrors) template override.

Note that the logic by which payments are matched to pledges is by looking at previous contributions in the series. The logic for memberships appears to be a combination of this & recording against the civicrm_membership record (some payment processors / code paths seem to implement the former - although the latter is the formal design, I don't know the relative advantages of the membership design vs the non-designed method)