Reference: hashicorp/terraform-plugin-go#351

The next versions of the plugin protocol (5.5/6.5) include support for provider defined functions. This change includes initial implementation of that support including:

- Temporarily pointing at terraform-plugin-go with provider function support (will be pointed at final terraform-plugin-go release before merge)
- New `function` package with all exposed Go types for provider developers to implement provider functions
- New `diag` package support for diagnostics with optional function argument information
- Implementation of new `GetFunctions` and `CallFunction` RPCs in the internal framework server, protocol 5/6 servers, and data handling between all layers
- Initial website documentation

This functionality may be released as experimental without compatibility promises until the protocol and Terraform's handling of provider functions is finalized. In that situation, all Go and website documentation will include additional callouts about the experimental nature of the functionality.

…) methods

…ts/attributes

The `AllowNullValue` parameter setting only prevents the value itself from being null, not elements/attributes.

…tribute value testing

…ment/attribute testing

…1460

Co-authored-by: Austin Valle <[email protected]>

Co-authored-by: Austin Valle <[email protected]>

Co-authored-by: Austin Valle <[email protected]>

…unctions

…ng zero-based and how to handle variadic parameters

Co-authored-by: Austin Valle <[email protected]>

…in-framework into functions