fuzz: add oss-fuzz build script

The interactions between value marks and unknown value refinements can be
a little tricky, so this pair of new tests cover two examples of that
interaction that are currently working and ought to stay that way.

The interactions between value marks and unknown value refinements can be
a little tricky, so this new addition to the "RefineWith" tests confirms
that it does indeed handle marked values correctly when passing through
the refinement spec.

When attempting to determine the final length range for a conditional
expression with collections, the length values may still be unknown.
Always use `Range()` to get the lower and upper bounds.

Correct mark handling for some conditional values.

Find correct refinement for overlapping ranges which could not have been
compared with `GreaterThan`. Also map inclusive flags for numeric
ranges.

Correct handling of DefinitelyNotNull collections.

Return a known null early when both conditional branches are null.

…inements

hclsyntax: conditional refinements of collections must use Range()

Callers might have additional rules for what's acceptable in a for_each
value for a dynamic block. For example, Terraform wants to forbid using
sensitive values here because it would cause the expansion to disclose the
length of the given collection.

Therefore this provides a hook point for callers to insert additional
checks just after the for_each expression has been evaluated and before
any of the built-in checks are run.

This introduces the "functional options" pattern for ExpandBlock for the
first time, as a way to extend the API without breaking compatibility with
existing callers. There is currently only this one option.

When hcldec needs to return a synthetic value, it must use
WithoutOptionalAttributesDeep to ensure the correct type for Null or
Unknown values. Optional attributes are normally removed from the value
type when decoding, but since hcldec is dealing with the decoder spec
directly, the optional attr types are still going to be present in these
cases.

…tesDeep

hcldec must use WithoutOptionalAttributesDeep