Support Terraform 0.15 schema

[radeksimko]

Terraform 0.15.0-beta1 was released recently and we need to check whether the schema differs from 0.14 - i.e. whether there are any deprecations or new attributes/blocks or ways the existing blocks/attributes are interpreted.

[radeksimko]

Addressed by a number of PRs:

• Add schema for Terraform 0.15 #24
• Reflect v0.15 configuration_aliases in provider referencing #26
• Add 0.15 terraform language attribute #29
• Support attribute nested type #31
• build(deps): bump github.com/hashicorp/terraform-json from 0.8.0 to 0.9.0 #32
• build(deps): bump github.com/hashicorp/terraform-json from 0.9.0 to 0.10.0 #34