proxy: add flag to enable http pool for all users (#5959)

## Problem

#5123

## Summary of changes

Add `--sql-over-http-pool-opt-in true` default cli arg. Allows us to set
`--sql-over-http-pool-opt-in false` region-by-region

test_runner/fixtures/neon_fixtures.py

@@ -2114,6 +2114,7 @@ class NeonProxy(PgProtocol):
                 # Console auth backend params
                 *["--auth-backend", "console"],
                 *["--auth-endpoint", self.endpoint],
+                *["--sql-over-http-pool-opt-in", "false"],
             ]
             if self.fixed_rate_limit is not None:
                 args += [