feat: add pgdog psql CLI subcommand

[murex971]

Adds a psql subcommand that opens an interactive psql shell (or runs a psql command) directly against a backend database defined in pgdog.toml.

Usage

# interactive shell
pgdog psql --database prod --user pgdog
# run a single statement
pgdog psql --database prod --user pgdog -- -c "SELECT version()"
# run a SQL file
pgdog psql --database prod --user pgdog -- -f migration.sql
# connect to a specific shard
pgdog psql --database prod --user pgdog --shard 1

Any flags after -- are forwarded verbatim to psql.

closes #430

[codecov]

Codecov Report

❌ Patch coverage is 0% with 81 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/cli.rs	0.00%	70 Missing ⚠️
pgdog/src/main.rs	0.00%	11 Missing ⚠️

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[meskill]

that'll break the compilation on non unix platforms. Although, it's not our main target we'd prefer the pgdog to be buildable on most platforms.
Let's think how we can avoid this platform-specific features or gate it behind cfg