[dbt-postgres] Please provide way to use Postgres connection string

### Is this your first time submitting a feature request?

- [x] I have read the [expectations for open source contributors](https://docs.getdbt.com/docs/contributing/oss-expectations)
- [x] I have searched the existing issues, and I could not find an existing issue for this feature
- [x] I am requesting a straightforward extension of existing dbt functionality, rather than a Big Idea better suited to a discussion

### Describe the feature

Please add a possibility to use Postgres connection string

Postgres server provides dozens ways to connect and authorize. It seems, all of them are available only if [Keyword/Value Connection Strings](https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-CONNSTRING) is used. And this is also a simpliest way - just ask user for a connection string and use it!

At the moment I currently being offered an inappropriate connection method (I am using a Unix domain socket and not use password to connect to Postgres server)
```
[...]
18:20:58 Setting up your profile.
Which database would you like to use?
[1] postgres

(Don't see the one you want? https://docs.getdbt.com/docs/available-adapters)

Enter a number: 1
host (hostname for the instance):
host (hostname for the instance): localhost
port [5432]:
user (dev username):
user (dev username): username
pass (dev password):

```

### Who will this benefit?

Postgres users