feature: support XDG Base Directory Specification on Unix

[brualan]

This PR adds optional support for the XDG Base Directory Specification on Unix-like systems for Terraform CLI configuration files. Specifically:

• If $XDG_CONFIG_HOME/terraform exists (as a directory), Terraform will load:
  • terraformrc from $XDG_CONFIG_HOME/terraform/terraformrc
  • provider/plugin config from $XDG_CONFIG_HOME/terraform/terraform.d
• If that directory does not exist, Terraform falls back to the legacy paths:
  • ~/.terraformrc
  • ~/.terraform.d

This approach ensures full backward compatibility: existing users and automation workflows remain unaffected. Only users who explicitly create the XDG config directory ($XDG_CONFIG_HOME/terraform) will use the new layout -- aligning with the principle of “don’t break existing setups.”

The implementation leverages Go’s standard library (os.UserConfigDir()) for correctness and avoids hardcoded paths. Comprehensive tests verify behavior under multiple scenarios:

• Empty home directory → legacy path
• XDG env set but no terraform/ dir → legacy path
• XDG terraform/ dir exists → XDG path used

This change addresses long-standing community feedback (see issue #15389) and helps reduce dotfile clutter in $HOME, bringing Terraform in line with modern Unix conventions.

Fixes #15389

Target Release

1.15.x

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

No changes to security controls.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.

## ENHANCEMENTS

- `cli`: Added optional support for XDG Base Directory Specification on Unix systems. Terraform will now use `$XDG_CONFIG_HOME/terraform/{terraformrc,terraform.d}` if the `terraform` directory exists, otherwise falling back to `~/.terraformrc` and `~/.terraform.d` for full backward compatibility. ([#15389](https://github.com/hashicorp/terraform/issues/15389))

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[crw]

Thanks for this submission. Please sign the CLA if you would like this to be in a reviewable state, and I will raise this PR in triage.

[brualan]

Thank you! I’ve just signed the CLA

[rvpert]

Hi @crw thanks for your work

I have been waiting 8 years for XDG support. Please finish this

[crw]

Apologies, I was under the impression this was getting follow-up from a maintainer. Let me kick that off now. Thanks for the bump.