Automatically enable tunnel for "tsh proxy db"

[greedy52]

changelog: automatically enables "--tunnel" for "tsh proxy db" when local tunnel is required

before:

$ tsh proxy db dynamo-table-a --db-user steve-poweruser
ERROR: "tsh proxy db" is not supported when:
  - database protocol is DynamoDB.

Please use the following command to connect to the database:
    tsh proxy db --tunnel dynamo-table-a

after:

$ tsh proxy db dynamo-table-a --db-user steve-poweruser -p 8888
Note: "--tunnel" flag has been automatically enabled when:
  - database protocol is DynamoDB.
To avoid this note, please add the "--tunnel" flag to this "tsh proxy db" command.

Started authenticated tunnel for the DynamoDB database "dynamo-table-a" in cluster "teleport.dev.aws.stevexin.me" on 127.0.0.1:8888.