Core: Add missing encryption keys when commiting snapshot

[ebyhr]

The added test fails if we revert the change to SnapshotProducer:

Expecting ArrayList:
  []
to contain:
  ["7Y+WYJxU4ff2Tq1U3mrLBg=="]
but could not find the following element(s):
  ["7Y+WYJxU4ff2Tq1U3mrLBg=="]

	at org.apache.iceberg.TestSnapshotProducerWithEncryption.propagateEncryptionKeysToMetadata(TestSnapshotProducerWithEncryption.java:66)
	at java.base/java.lang.reflect.Method.invoke(Method.java:580)

EncryptionUtil.decryptManifestListKeyMetadata throws NPE if we remove the assertion at L66:

  "java.lang.NullPointerException: Cannot invoke "org.apache.iceberg.encryption.EncryptedKey.encryptedKeyMetadata()" because "manifestListKey" is null
	at org.apache.iceberg.encryption.EncryptionUtil.decryptManifestListKeyMetadata(EncryptionUtil.java:153)
	at org.apache.iceberg.BaseManifestListFile.decryptKeyMetadata(BaseManifestListFile.java:47)
	at org.apache.iceberg.encryption.EncryptingFileIO.newInputFile(EncryptingFileIO.java:125)
	at org.apache.iceberg.BaseSnapshot.cacheManifests(BaseSnapshot.java:187)
	at org.apache.iceberg.BaseSnapshot.dataManifests(BaseSnapshot.java:213)
	at org.apache.iceberg.DataTableScan.doPlanFiles(DataTableScan.java:68)
	at org.apache.iceberg.SnapshotScan.planFiles(SnapshotScan.java:162)
	at org.apache.iceberg.TestSnapshotProducerWithEncryption.lambda$propagateEncryptionKeysToMetadata$0(TestSnapshotProducerWithEncryption.java:65)