PGDumpCmd is the path to the `pg_dump` executable.

Error can ship a cmd output as well as the start interface.

ExportResult is the result of an export operation..

Postgres is an `Exporter` interface that backs up a Postgres database via the `pg_dump` command.

S3 is a `Storer` interface that puts an ExportResult to the specified S3 bucket.

Exporter is expected to export "something" to a file and return a complete `ExportResult` struct (`Path`, `MIME`, `Error`).

Storer takes an `ExportResult` and move it somewhere! To a cloud storage service, for instance...