* GH-3043: Add FileHeaders.REMOTE_HOST header
Fixes https://github.com/spring-projects/spring-integration/issues/3043
* Populate a `FileHeaders.REMOTE_HOST` from the
`AbstractRemoteFileStreamingMessageSource` and "get"-based commands
in the `AbstractRemoteFileOutboundGateway`
* Extract the value from the a `Session.getHost()` contract
* The `AbstractInboundFileSynchronizingMessageSource` cannot be
addressed with this because the real message is already based on the
locally stored file
* Adjust some affected tests according our code style requirements
* * Add remote file info support into `AbstractInboundFileSynchronizingMessageSource`
* Introduce a `MetadataStore` functionality into the `AbstractInboundFileSynchronizer`
to gather a remote file info an save it in the URI style against local file
* Retrieve such an info in the `AbstractInboundFileSynchronizingMessageSource`
during local file polling
* Introduce `protocol()` contract for the `AbstractInboundFileSynchronizer`
to build a proper URI in the metadata for external readers to distinguish
remote files properly
* Document the feature
* * Fix some typos in Docs
* * Rename property and header constant to the `HOST_PORT` pair
* Fix typos in Docs
* Add `remote-file-metadata-store` and `metadata-store-prefix` into XSD
of (S)FTP Inbound Channel Adapters
* Add `remoteFileMetadataStore` and `metadataStorePrefix` options
into `RemoteFileInboundChannelAdapterSpec` for Java DSL
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