allow configuring the Config directly, without having to call `ParseConfig`

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**Is your feature request related to a problem? Please describe.**
I work on software where the user fills in the postgres configuration into a form. then i have to construct a connection-string from the data i have (not to forget to escape stuff etc.), and then call `ParseConfig`.

**Describe the solution you'd like**
A clear and concise description of what you want to happen.
I'd like to be able to fill out a go structure (`pgconn.Config`?) with the info i already collected from the user directly, without having to consruct the connection-string.

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
Ideally this approach would also skip all the "compatible with C' things, like looking for stuff in default-places, read environment variables etc. but that may be a separate feature request.

Start by reading the existing ParseConfig entry point and the pgconn.Config type. Trace how connection-string parsing supplies configuration, then determine the scope of a direct-configuration path; skipping defaults and environment variables is identified as a possible separate concern.

Written by the indexing model from the issue text.