This change is an attempt at bridging the gap between kurtosis devnets
and future alphanet/betanet specification.
(see ethereum-optimism/devnets#4)

It does so by defining specific override points in a more generic
template.
Generic building blocks are defined in templates/, and rely on a new
"include" capability at the template level.

This approach might also provide a somewhat lighter entry point for
user devnet definitions, following the same principle.

Note that this is completely optional at this point, and the full
scope of the kurtosis definition is still available to whomever
needs/wants it.