feat(sftp): add modtime as metadata. [#2955]

[danriedl]

This PR adds:

• feat(sftp): add modtime as metadata. [Enhance the SFTP input component to provide modification time as metadata #2955]

[Jeffail]

Hey @danriedl, this could end up with quite a large performance impact when there's many messages coming from individual files (unless there's some underlying caching during the stat calls). It's probably better to extract this info in the same place where we capture the currentPath, so that it's only performed once per file.

If we add more information over time it'll start looking a bit messy having all of these fields added to the sftpReader type so it'd also be good to add some sort of currentFileInfo struct that carries all of this info.