Optional type cost handlers

[TristonianJones]

The cost estimation and runtime cost tracking accounts for standard library
functions and a handful of extension functions; however, the optional types
which use special cased short-circuiting and custom selection / indexing
methods are not properly observed by either the check-time or runtime
components.

The first step is to introduce cost estimation extensions into a future iteration
of the cel.OptionalTypes() library; however, this is not quite enough given
the internal features of hint-path generation and entry size estimation which
happen during the check pass