[BUG]release-1.9 branch does not support startupmode of "external subscription"

[mmaxiaolei]

Describe the bug
release-1.9 branch does not support startupmode of "external subscription".
PulsarTableSink support "external subscription". but Pulsarvalidator disables this mode：

pulsar-flink/pulsar-flink-connector/src/main/java/org/apache/flink/table/descriptors/PulsarValidator.java

Line 116 in 67852f4

public static String normalizeStartupMode(StartupMode startupMode) {

To Reproduce

Expected behavior
release-1.9 branch support startupmode of "external subscription"

Screenshots

Additional context

[syhily]

Tks for your report.