cmd: make dsn a non-mandatory flag to allow use of Postgres environment variables

[scottt]

Description

psql and programs using libpq allows users to use the environment variables PGHOST and friends to specify the connection information. This PR implements the same behavior for pg-schema-diff by making the DSN flag non-mandatory.

Motivation

• We already set the Postgres environment variables in our development and deployment environments.
• The Javascript and Golang libraries we use honor the variables.
• This change makes pg-schema-diff "just work" in our workflow.

Implementation

• pg-schema-diff's (connFlags).parseConnConfig() calls pgx.ParseConfig(), which calls pgconn.ParseConfigWithOptions()

• pgconn.ParseConfigWithOptions already reads the Postgres environment variables.

Testing

• I've tested the change manually on the command line and it works as expected

[CLAassistant]

All committers have signed the CLA.

[scottt]

@bplunkett-stripe , I've incorporated the comments in f3c5658 which makes pg-schema-diff output:

2023/08/10 17:33:02 [WARNING] DSN flag not set. Using libpq environment variables and default values.

I've again manually tested pg-schema-diff from the command line and the new warning and database connection works as expected.