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/server-config.yaml.golden

@@ -483,6 +483,14 @@ ephemeralDeployment: false
 # authentication (awsiamrds) is recommended.
 # (default: password, type: enum[password\|awsiamrds])
 pgAuth: password
+# Maximum number of open connections to the database. Defaults to 10.
+# (default: 10, type: int)
+pgConnMaxOpen: 10
+# 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.
+# (default: auto, type: string)
+pgConnMaxIdle: auto
 # A URL to an external Terms of Service that must be accepted by users when
 # logging in.
 # (default: <unset>, type: string)