Add an include_indented tag for indented template includes

Add an include_indented tag for indented template includes

jinja2's {% include %} inserts the rendered file verbatim, so includes
inside view statements have to be wrapped in a three-line
{% filter indent %} block with the depth repeated by hand. Add a small
jinja2 extension providing {% include_indented "..." %}, which detects
the tag's own leading whitespace at parse time and expands to the
builtin include wrapped in the builtin indent filter, keeping stock
jinja2 runtime semantics. Detection needs the tag alone on its line in
a loader-backed template; anything else fails at parse time instead of
misrendering.

Co-Authored-By: Martin Basti <mbasti@isc.org>
Assisted-by: Claude:claude-fable-5