Updates and refactors the bigip pool module to use REST (#25234)

This patch updates the bigip_pool module to use F5's REST API
as well as includes unit tests.

Unit tests are provided. Integration tests can be found here

https://github.com/F5Networks/f5-ansible/blob/devel/test/integration/bigip_pool.yaml#L23
https://github.com/F5Networks/f5-ansible/tree/devel/test/integration/targets/bigip_pool/tasks

test/units/modules/network/f5/fixtures/load_ltm_pool.json

@@ -0,0 +1,32 @@
+{
+  "kind": "tm:ltm:pool:poolstate",
+  "name": "test_pool",
+  "partition": "Common",
+  "fullPath": "/Common/test_pool",
+  "generation": 1452,
+  "selfLink": "https://localhost/mgmt/tm/ltm/pool/~Common~test_pool?ver=11.5.4",
+  "allowNat": "yes",
+  "allowSnat": "yes",
+  "description": "test",
+  "ignorePersistedWeight": "disabled",
+  "ipTosToClient": "pass-through",
+  "ipTosToServer": "pass-through",
+  "linkQosToClient": "pass-through",
+  "linkQosToServer": "pass-through",
+  "loadBalancingMode": "round-robin",
+  "minActiveMembers": 0,
+  "minUpMembers": 0,
+  "minUpMembersAction": "failover",
+  "minUpMembersChecking": "disabled",
+  "monitor": "min 1 of { /Common/http /Common/inband }",
+  "queueDepthLimit": 0,
+  "queueOnConnectionLimit": "disabled",
+  "queueTimeLimit": 0,
+  "reselectTries": 0,
+  "serviceDownAction": "reselect",
+  "slowRampTime": 10,
+  "membersReference": {
+    "link": "https://localhost/mgmt/tm/ltm/pool/~Common~test_pool/members?ver=11.5.4",
+    "isSubcollection": true
+  }
+}

test/units/modules/network/f5/fixtures/pool_members_subcollection.json

@@ -0,0 +1,21 @@
+[
+    {
+      "kind": "tm:ltm:pool:members:membersstate",
+      "name": "1.1.1.1:80",
+      "partition": "Common",
+      "fullPath": "/Common/1.1.1.1:80",
+      "generation": 1,
+      "selfLink": "https://localhost/mgmt/tm/ltm/pool/~Common~test_pool/members/~Common~1.1.1.1:80?ver=11.5.4",
+      "address": "1.1.1.1",
+      "connectionLimit": 0,
+      "dynamicRatio": 1,
+      "inheritProfile": "enabled",
+      "logging": "disabled",
+      "monitor": "default",
+      "priorityGroup": 0,
+      "rateLimit": "disabled",
+      "ratio": 1,
+      "session": "user-disabled",
+      "state": "up"
+    }
+  ]