Add BlockMarshaler and ExpressionMarshaler interfaces

[ropnop]

Hello! This addresses my feature request in #607 . Internally we have a need for this as we want to be able to implement custom types that either emit raw, unescaped HCL, and be able to encode map[string]string to HCL easily.

These 2 interfaces let us implement the custom marshalling logic in our code while introducing minimal changes to this library code.

I added a few tests in encode_test.go to demonstrate using these interfaces to overwrite expression and block marshaling behavior on custom types, as well as some tests for handling pointer, nil and zero cases (nil and zero will panic - i think this desired behavior)

Let me know what you think - thanks!

[hashicorp-cla]

All committers have signed the CLA.

[yuri-tceretian]

Great feature! That can make encoding to HCL really powerful and let us implement custom encoding the same way we do JSON\YAML marshallers.

It would be great if maintainers could take a look and eventually merge it.

[mickeypash]

@ansgarm what are the steps to getting this merged?

[crw]

Hi, we discussed this in triage today. This looks like a feature that we would be willing to review. The main barrier is the time it would take to understand why it was not implemented this way originally, to make sure we were not inadvertently introducing new issues by committing this change. Unfortunately, this comes down to maintainer availability. Hopefully the next time someone is doing a deep dive into HCL, we can pick this back up. Thanks!