gohcl: enable customizing decoding with decode options

[jsternberg]

Adds a struct DecodeOptions that can be used to customize some of the behavior of DecodeBody and DecodeExpression.

This allows customizing the behavior of determining the implied type from a Go type and customizing the behavior of converting between cty types.

This is part of some work for the docker buildx bake command: docker/buildx#2758

We'd like some specialized convert logic so we can have a type be either in a string format or a more structured object format. The easiest way we've found to do this is to utilize capsule types and override the ImpliedType function. We can then add a custom ImpliedType function to recognize specific types and insert a capsule type instead of the default implied type.

While this change also allows customizing Convert, I found that method much harder to modify. The implementation was much more involved and would require more code to be duplicated due to nested types.

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