Migrate provider to terraform-plugin-framework

[mraerino]

The terraform-plugin-sdk used in this provider has some limitations when it comes to newer provider features.

I think it could be beneficial to the project to eventually switch to the newer terraform-plugin-framework.

An example of something that's not possible with terraform-plugin-sdk is nested schemas (issue), which would help a lot with non-string values for custom fields (#349) and general ergonomics of custom fields.

There seems to be a way to use tf5muxserver to gradually migrate a provider to the plugin framework.