supports the feature extra credentials

it starts to support the idea of extra credentials where it allows prestosql client user to provide DB2 username and password as extra credentials that are passed directly to the backend DB2 server when running a Presto query.

1. configure this for the DB2 connector catalog properties file:

user-credential-name=db2_user
password-credential-name=db2_password

2. passing credentials directly to DB2 server: $ presto --extra-credential db2_user=user1 --extra-credential db2_password=secret

first release after switching to prestosql.io

Follow-up of #12

0.215

Upgrades to Presto 0.215

0.193

First release 0.193