Allow importing members

re #17 and #29

I'm not sure how to add a test for `import`ing yet.
The cool tf harness doesn't support `import`.

The terraform file I used for testing.

```
terraform {
  required_providers {
    zerotier = {
      source  = "zerotier/zerotier"
    }
  }
}
resource "zerotier_member" "bob" {}
```

`terraform import zerotier_member.bob 8286ac0e4786b2aa-1122334455` The member needs to already exist

`terraform state show zerotier_member.bob`

implementation borrowed from bltavares  branch

docs/resources/member.md

@@ -46,3 +46,11 @@ resource "zerotier_member" "alice" {
 - **no_auto_assign_ips** (Boolean) Exempt this member from the IP auto assignment pool on a Network
 
 
+
+## Import
+
+Members can be imported using their Network ID and Node ID, for example:
+
+```
+$ terraform import zerotier_network.network "8056c2e21c1930be-1122334455"
+```

pkg/zerotier/resource_member.go

@@ -2,6 +2,7 @@ package zerotier
 
 import (
 	"context"
+	"strings"
 
 	"github.com/hashicorp/terraform-plugin-sdk/v2/diag"
 	"github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema"
@@ -16,6 +17,9 @@ func resourceMember() *schema.Resource {
 		UpdateContext: resourceMemberUpdate,
 		DeleteContext: resourceMemberDelete,
 		Schema:        MemberSchema,
+		Importer: &schema.ResourceImporter{
+			State: schema.ImportStatePassthrough,
+		},
 	}
 }
 
@@ -26,7 +30,9 @@ func resourceMember() *schema.Resource {
 func resourceMemberRead(ctx context.Context, d *schema.ResourceData, m interface{}) diag.Diagnostics {
 	c := m.(*ztcentral.Client)
 
-	member, err := c.GetMember(ctx, d.Get("network_id").(string), d.Get("member_id").(string))
+	nwid, nodeId := resourceNetworkAndNodeIdentifiers(d)
+
+	member, err := c.GetMember(ctx, nwid, nodeId)
 	if err != nil {
 		return diag.FromErr(err)
 	}
@@ -75,3 +81,14 @@ func resourceMemberDelete(ctx context.Context, d *schema.ResourceData, m interfa
 	d.SetId("")
 	return nil
 }
+
+func resourceNetworkAndNodeIdentifiers(d *schema.ResourceData) (string, string) {
+	nwid := d.Get("network_id").(string)
+	nodeID := d.Get("member_id").(string)
+
+	if nwid == "" && nodeID == "" {
+		parts := strings.Split(d.Id(), "-")
+		nwid, nodeID = parts[0], parts[1]
+	}
+	return nwid, nodeID
+}