Add support for is_alphanumeric in autolink reference

[mmh]

GitHub recently added an option for is_alphanumeric in github_repository_autolink_reference. This was added to v5.8.0 of terraform-github-provider and documented here

Bumping the provider causes a change to all our current autolinks, like below.

-/+ resource "github_repository_autolink_reference" "repository_autolink_reference" {
      + etag                = (known after apply)
      ~ id                  = "346367" -> (known after apply)
      + is_alphanumeric     = true # forces replacement
        # (3 unchanged attributes hidden)
  }

It would be great if autolink_references could be extented to support setting is_alphanumeric.

[tobiasehlert]

Created PR #156 that adds possibility to set is_alphanumeric inside autolink_reference to false.