Allow to dump data only

There is an `edgedb dump` command that does a full dump of the database(s).
`edgedb restore` command could take a generated dump file(s) and apply them to the empty database. However, there is no way to dump (or restore) data only.

What did I try?
1. A `*.dump` file provides the EdgeQL code for the schema, however, keeps data in binary format - no way to apply those data.
2. https://www.edgedb.com/docs/changelog/3_x#sql-support states the SQL support. However, using `pg_dump` gives a relation representation of the data that is (let’s say) impossible to apply to an EdgeDB instance.

As a solution, I propose to add `--data-only` option to the `edgedb dump` command that allows getting EdgeQL data insert queries.

There is also the related question: why does `edgedb dump` keeps data in the binary form? As far as I see, a `—format binary|edgeql` option will make sense to provide a consistent output regardless of what’s being dumped: all, data only, etc.