feat: make database connection pool size configurable (#21403)

Closes https://github.com/coder/coder/issues/21360

A few considerations/notes:
- I've kept the number of conns to 10 in all other places, except coderd
- which uses the config value
- I opted to also make idle conns configurable; the greater the delta
between max open and max idle, the more connection churn
- Postgres maintains a [_process_ per
connection](https://www.postgresql.org/docs/current/connect-estab.html),
contrary to what the comment said previously
- Operators should be able to tune this, since process churn can
negatively affect OS scheduling
- I've set the value to `"auto"` by default so it's not another knob one
_has to_ twiddle, and sets max idle = max conns / 3

---------

Signed-off-by: Danny Kopping <danny@coder.com>

cli/testdata/coder_server_--help.golden

@@ -65,6 +65,14 @@ OPTIONS:
           Type of auth to use when connecting to postgres. For AWS RDS, using
           IAM authentication (awsiamrds) is recommended.
 
+      --postgres-conn-max-idle string, $CODER_PG_CONN_MAX_IDLE (default: auto)
+          Maximum number of idle connections to the database. Set to "auto" (the
+          default) to use max open / 3. Value must be greater or equal to 0; 0
+          means explicitly no idle connections.
+
+      --postgres-conn-max-open int, $CODER_PG_CONN_MAX_OPEN (default: 10)
+          Maximum number of open connections to the database. Defaults to 10.
+
       --postgres-url string, $CODER_PG_CONNECTION_URL
           URL of a PostgreSQL database. If empty, PostgreSQL binaries will be
           downloaded from Maven (https://repo1.maven.org/maven2) and store all