Add support for 'Partition' and 'RetryJoin' (#12126)

- Adding a 'Partition' and 'RetryJoin' command allows test cases where
  one would like to spin up a Consul Agent in a non-default partition to
test use-cases that are common when enabling Admin Partition on
Kubernetes.

.changelog/12126.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+sdk: Add support for `Partition` and `RetryJoin` to the TestServerConfig struct.
+```

sdk/testutil/server.go

@@ -77,6 +77,8 @@ type TestServerConfig struct {
 	Performance         *TestPerformanceConfig `json:"performance,omitempty"`
 	Bootstrap           bool                   `json:"bootstrap,omitempty"`
 	Server              bool                   `json:"server,omitempty"`
+	Partition           string                 `json:"partition,omitempty"`
+	RetryJoin           []string               `json:"retry_join,omitempty"`
 	DataDir             string                 `json:"data_dir,omitempty"`
 	Datacenter          string                 `json:"datacenter,omitempty"`
 	Segments            []TestNetworkSegment   `json:"segments"`