use hclmerge to merge a new tflint config file when there is override hcl config

[lonegunmanb]

In the past we'll use .tflint_alt.hcl instead of downloaded avm.tflint.hcl, so the repo's owner could customize their own tflint config. But once we've upgraded the avm.tflint.hcl in the center repo, we might need upgrade all .tflint_alt.hcl files to adapt the new config, and that could be very difficult.

In this pr I use hclmerge to ease the pain. It could merge two hcl files like Terraform override file, so when the repo owners want to customize a config, they don't need to maintain a complete .tflint_alt.hcl file anymore, instead, they could create a avm.tflint.override.hcl or avm.tflint_example.override.hcl file, contains the parts they'd like to patch on the centralized version config file.