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>

coderd/apidoc/docs.go

@@ -14380,6 +14380,12 @@ const docTemplate = `{
                 "pg_auth": {
                     "type": "string"
                 },
+                "pg_conn_max_idle": {
+                    "type": "string"
+                },
+                "pg_conn_max_open": {
+                    "type": "integer"
+                },
                 "pg_connection_url": {
                     "type": "string"
                 },

coderd/apidoc/swagger.json

@@ -12959,6 +12959,12 @@
 				"pg_auth": {
 					"type": "string"
 				},
+				"pg_conn_max_idle": {
+					"type": "string"
+				},
+				"pg_conn_max_open": {
+					"type": "integer"
+				},
 				"pg_connection_url": {
 					"type": "string"
 				},