PostgresClient should support additionalStartupParameters

[thoven87]

Is your feature request related to a problem? Please describe.
Allow options to set the Application names for each connection/Query

Describe the solution you'd like
PostgresClient.Configuration should accept application name

It makes debuting which application querying the database for debug/audit purposes

Active queries can be retrieves easily with

SELECT 
    pid
    datname,
    usename,
    application_name,
    client_hostname,
    client_port,
    backend_start,
    query_start,
    query,
    state
FROM pg_stat_activity
WHERE state = 'active';

Describe alternatives you've considered
Alternatively, a connection string should be allow to set the application instead of calling
postgresql://username@localhost/database?application_name=my_application

withConnection{ connection in
 connection.query("SET application_name TO my_application")
let stream = connection.query("Some query")
}
`

**Additional context**
This is very useful incases where multiple services can access the same database.

[gwynne]

@fabianfett Looks like this is available through the old PostgresConnection configuration but not through PostgresClient's.

[fabianfett]

@thoven87 sorry for the very slow response. You can achieve this with todays PostgresConnection API:

var configuration = PostgresConnection.Configuration(username: "my_username", tls: .disable)
configuration.options.additionalStartupParameters = [("application_name", "my_application")]

As @gwynne points out, this is not possible with today's PostgresClient API. However it should be fairly trivial to add this behavior.

[fabianfett]

@thoven87 PR raised as #521.

[thoven87]

@thoven87 PR raised as #521.

Thank you! I'll take a look soon!