Pmm-agent changes regarding new Query source - pg_stat_monitor

TBD

Suggested Implementation:

• We should implement new QAN agent which will be like PostgreSQLStatStatementsAgent.
• Add new agent type to supervisor

DOD:

• Once PMM-agent get request from pmm-managed to run pg_stat_monitor Agent, it connects to pg_stat_monitor, collects data and sends collected data to pmm-managed.
• pg_stat_monitor Agent should be able to collect query examples
  • If disable_query_examples is passed the agent should not collect query examples

Useful links: 

• https://github.com/percona/percona-docker/tree/master/percona-distribution-postgresql-11
• https://github.com/percona/pg_stat_monitor
• https://docs.google.com/document/d/1lXPZKDNs_1qO0lY1tZVJvmawjwcxABocv5MWzh5yWUM/edit# https://confluence.percona.com/display/PMM/Add+Percona+pg_stat_monitor+as+second+PostgreSQL+query+data+source#AddPerconapg_stat_monitorassecondPostgreSQLquerydatasource-Techdocumentationfortheplugin [   
  https://docs.google.com/document/d/1lXPZKDNs_1qO0lY1tZVJvmawjwcxABocv5MWzh5yWUM/edit#|https://docs.google.com/document/d/1lXPZKDNs_1qO0lY1tZVJvmawjwcxABocv5MWzh5yWUM/edit#]