Add support for format versions 1.0 and above

[alisdair]

A future release of Terraform will ship with a 1.0 format version for all stable JSON formats, which will be backwards compatible with the existing format. This will also mark a release where the JSON compatibility guarantee is more clearly defined, and so format versions less than 2.0 should always be backwards compatible with consumers which support 1.0.

This commit prepares for that change by adding support for version constraints rather than supported fixed sets of versions. This is done using the go-version package.

Includes test data from a pre-release of Terraform 1.1 which includes the planned JSON format version changes (this branch).

[radeksimko]

This looks pretty good to me! Thank you for raising the PR.

I left some relatively minor comments in-line.

The next logical step would be perhaps trying it out in the context of terraform-exec where you can run the E2E tests against a custom build via

TFEXEC_E2ETEST_VERSIONS=refs/heads/alisdair/json-format-version-1.0 go test ./tfexec/internal/e2etest/...

[alisdair]

I tried running terraform-exec tests against the relevant branches, and it fails in what I think is a reasonable way:

$ go mod edit -replace github.com/hashicorp/terraform-json=/Users/alisdair/code/terraform-json
$ TFEXEC_E2ETEST_VERSIONS=refs/heads/alisdair/json-format-version-1.0 go test ./tfexec/internal/e2etest/...
2021/09/14 14:21:29
--- FAIL: TestProvidersSchema (1.63s)
    --- FAIL: TestProvidersSchema/0_basic (1.43s)
        --- FAIL: TestProvidersSchema/0_basic/basic-refs/heads/alisdair/json-format-version-1.0 (1.43s)
            util_test.go:112: [INFO] running Terraform command: /var/folders/2p/ys5rmmyd02b4f74gzxlv5nj00000gp/T/tfinstall607081317/gitref-refs-heads-alisdair-json-format-version-1.0/terraform2149754593 init -no-color -force-copy -input=false -backend=true -get=true -upgrade=false
            util_test.go:112: [INFO] running Terraform command: /var/folders/2p/ys5rmmyd02b4f74gzxlv5nj00000gp/T/tfinstall607081317/gitref-refs-heads-alisdair-json-format-version-1.0/terraform2149754593 providers schema -json -no-color
            providers_schema_test.go:324: mismatch (-want +got):
                  &tfjson.ProviderSchemas{
                - 	FormatVersion: "0.2",
                + 	FormatVersion: "1.0",

(and some similar errors).

At first glance it seems like updating those assertions accordingly would be a reasonable step once 1.1 is released.