TUN-7134: Acquire token for cloudflared tail

cloudflared tail will now fetch the management token from by making
a request to the Cloudflare API using the cert.pem (acquired from
cloudflared login).

Refactored some of the credentials code into it's own package as
to allow for easier use between subcommands outside of
`cloudflared tunnel`.

cfapi/tunnel.go

@@ -50,6 +50,10 @@ type newTunnel struct {
 	TunnelSecret []byte `json:"tunnel_secret"`
 }
 
+type managementRequest struct {
+	Resources []string `json:"resources"`
+}
+
 type CleanupParams struct {
 	queryParams url.Values
 }
@@ -133,6 +137,28 @@ func (r *RESTClient) GetTunnelToken(tunnelID uuid.UUID) (token string, err error
 	return "", r.statusCodeToError("get tunnel token", resp)
 }
 
+func (r *RESTClient) GetManagementToken(tunnelID uuid.UUID) (token string, err error) {
+	endpoint := r.baseEndpoints.accountLevel
+	endpoint.Path = path.Join(endpoint.Path, fmt.Sprintf("%v/management", tunnelID))
+
+	body := &managementRequest{
+		Resources: []string{"logs"},
+	}
+
+	resp, err := r.sendRequest("POST", endpoint, body)
+	if err != nil {
+		return "", errors.Wrap(err, "REST request failed")
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode == http.StatusOK {
+		err = parseResponse(resp.Body, &token)
+		return token, err
+	}
+
+	return "", r.statusCodeToError("get tunnel token", resp)
+}
+
 func (r *RESTClient) DeleteTunnel(tunnelID uuid.UUID) error {
 	endpoint := r.baseEndpoints.accountLevel
 	endpoint.Path = path.Join(endpoint.Path, fmt.Sprintf("%v", tunnelID))