Fix Framework allows top-level schema attributes that conflict with Terraform meta-arguments

[bendbennett]

Closes: #136

Once all references to tfsdk.Schema have been removed we can ammend fwschema/schema.go:

type Schema interface {
    /* ... */

    Validate() diag.Diagnostics
}

[bflad]

Looks good to me 🚀 Just little things to followup on.

[bflad]

Since the nested attributes might be done using list, map, or set mode, we'll also need to account for adding those intermediate steps (AtListIndex, AtMapKey, AtSetValue) to the path. We're not looking at a real configuration though, so it's a little awkward to make it "correct", although Terraform likely won't show the source configuration matching the path information when it is returned by the GetProviderSchema RPC.

Let's create a followup issue, but we should likely try to fix up this path information and consider including it in the diagnostic details in case it is not in Terraform's output. Maybe we can use something like the zero-value (e.g. AtListIndex(0), AtMapKey(""), AtSetValue(/* ? */)) for describing those intermediate path steps for static validation.

[bendbennett]

#574

[bflad]

Similarly for block pathing and block nesting modes.

[bendbennett]

#574

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