Add @NullMarked package-info for cache-provider API

Signed-off-by: Soby Chacko <soby.chacko@broadcom.com>

Author: sobychacko

spring-pulsar-cache-provider/src/main/java/org/springframework/pulsar/cache/provider/package-info.java

@@ -0,0 +1,5 @@
+/**
+ * Package containing the cache provider abstraction for Pulsar.
+ */
+@org.jspecify.annotations.NullMarked
+package org.springframework.pulsar.cache.provider;