Add common parameter names as const

[domi4484]

Provide handy constants for common used parameters

[gacarrillor]

Looks good @domi4484

Out of curiosity, where would you like to use these constants?

[domi4484]

I would like to know the key names for the dict returned by service_config. At leas the most commons...

[gacarrillor]

I would like to know the key names for the dict returned by service_config. At leas the most commons...

Got it, could you add that to the comment above the parameter list?
Just to let future readers know about what are they for.

BTW, a complete list can be found here, just in case you want to add the URL to the comment, but could be too much :)
https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-PARAMKEYWORDS

[domi4484]

@gacarrillor thanks for pointing me to the doc. As there are so many parameters I don't think anymore that this is a useful addition. I'll close this PR