[Remote Store] Make uploadMetadata private and call it internally from RemoteSegmentStoreDirectory

[sachinpkale]

Describe the solution you'd like As per the comment: https://github.com/opensearch-project/OpenSearch/pull/4020#discussion_r939948386, uploadMetadata should not be a public method. This will allow to abstract it as part of RemoteSegmentStoreDirectory and callers will not have to understand how remoteMetadataDirectory works internally.