Expose marshaller interfaces for expressions and blocks

Hello -

I'd like to propose an enhancement that would allow me to use this library and extend with custom functionality in my own projects.

I would like to define 2 marshaler interfaces in the library that I can implement in my own project:

```
// ExpressionMarshaler defines an interface for marshaling a field type as an HCL expression.
type ExpressionMarshaler interface {
MarshalExpression() (*hclwrite.Expression, error)
}

// BlockMarshaler defines an interface for marshalling a type to an *hclwrite.Block
// It can be used to implement custom hcl block writing for structs and types, including
// when used as fields with the block tag
type BlockMarshaler interface {
MarshalBlock(blockType string) *hclwrite.Block
}
```

In the [EncodeAsBlock](https://github.com/hashicorp/hcl/blob/main/gohcl/encode.go#L63) and [populateBody](https://github.com/hashicorp/hcl/blob/main/gohcl/encode.go#L51) functions, I'd like to then check if the `interface{}` passed in implements these respective interfaces and if it does, just call the function.

This would allow other consumers of this library to extend/customize the marshalling behavior when using `hclwrite` for custom types and structs.

I have a working fork of this and happy to open a PR, but wanted to start a discussion here first if you are okay with this approach.