Support indirection for all shr-tag-* calls

Support indirection for all shr-tag-* calls

The 'shr-external-rendering-functions' variable was previously only
honored in the shr-descend function, now all direct calls to the
shr-tag-* functions have been replaced by a call to
'shr-indirect-call' which tries using an alternative rendering
function first.

* lisp/net/shr.el (shr-indirect-call): New helper function.
(shr-descend, shr-tag-object, shr-tag-video):
(shr-collect-extra-strings-in-table): Fix callers to call via
shr-indirect-call.  (Bug#28402)