hclwrite: Consider Attribute Rename Support

[bflad]

Description

I am currently working on a proof of concept tool that could help practitioners migrate their Terraform HCL configurations from null_resource resource blocks to terraform_data resource blocks (as well as adding moved blocks). Renaming the block types in-place works great, however similar functionality is not available for hclwrite.Attribute to update the attribute name in-place. Currently the workaround seems something like:

body.RemoveAttribute("triggers")
body.SetAttributeRaw("triggers_replace", triggersAttribute.Expr().BuildTokens(nil))

However that will cause the attribute and its expression to be moved to the bottom of the same block, causing unnecessary configuration churn.

Proposal

Similar to the (*hclwrite.Block).SetType() method added in #340, create a new (*hclwrite.Attribute).SetName() method, e.g.

func (*Attribute) SetName(name string)

That only replaces the underlying name node in the AST. I have verified this locally and will submit the small addition for consideration. Thank you!