Add qldb, qldbsession and small examples (#324)

* Add qldb, qldbsession and small examples

'Qldb' is an AWS database service. There are two webservices: qldb and
qldbsession. The former has APIs like Create, List and DeleteLedger. The
later is the dataplane API (like connecting to MySQL or Postgres).

QldbSession really has only 1 API, called "send command". A command is
something like "start a session", or "here is some sql".

The 'QldbSession' example isn't very useful, as customers would never
actually use the underlying API. Rather, they would use something like
the driver: https://github.com/awslabs/amazon-qldb-driver-rust

.

The 'Qldb' is a simple list-ledgers example, similar to what's found for
other services.

* Update aws/sdk/examples/qldb-list-ledgers/src/main.rs

Co-authored-by: Marc Bowes <bowes@amazon.com>
Co-authored-by: Russell Cohen <rcoh@amazon.com>