oxide_instance: add validation to ssh_public_keys attribute

Add validation to the `ssh_public_keys` attribute to validate that
it contains no empty or null values. A customer pointed out that the
following Terraform configuration resulted in an unclear error message.

```hcl
resource "oxide_instance" "example" {
  ssh_public_keys = ["4db0f0d3-8e82-491a-beff-39764be1c529", null]
}
```

Even worse, the error message didn't occur until apply time.

```
oxide_instance.example: Creating...
╷
│ Error: Error retrieving name or ID information
│
│   with oxide_instance.example,
│   on main.tf line 22, in resource "oxide_instance" "example":
│   22: resource "oxide_instance" "example" {
│
│ name or ID parse error: invalid syntax
╵
```

With this change, the error message occurs at plan time and is much
more clear.

```
> terraform apply
╷
│ Error: Null Set Value
│
│   with oxide_instance.example,
│   on main.tf line 31, in resource "oxide_instance" "example":
│   31:   ssh_public_keys  = ["4db0f0d3-8e82-491a-beff-39764be1c529", null]
│
│ This attribute contains a null value.
╵
```

Author: sudomateo

internal/provider/resource_instance.go

@@ -11,6 +11,7 @@ import (
 	"time"
 
 	"github.com/hashicorp/terraform-plugin-framework-timeouts/resource/timeouts"
+	"github.com/hashicorp/terraform-plugin-framework-validators/setvalidator"
 	"github.com/hashicorp/terraform-plugin-framework-validators/stringvalidator"
 	"github.com/hashicorp/terraform-plugin-framework/attr"
 	"github.com/hashicorp/terraform-plugin-framework/diag"
@@ -178,6 +179,12 @@ func (r *instanceResource) Schema(ctx context.Context, _ resource.SchemaRequest,
 				PlanModifiers: []planmodifier.Set{
 					setplanmodifier.RequiresReplace(),
 				},
+				Validators: []validator.Set{
+					setvalidator.NoNullValues(),
+					setvalidator.ValueStringsAre(
+						stringvalidator.NoneOf(""),
+					),
+				},
 			},
 			"network_interfaces": schema.SetNestedAttribute{
 				Optional:    true,