Add before_acquire passthrough and idle-ping shorthand to ConnectOptions (#3143)

Expose SQLx's `before_acquire` pool hook, which SeaORM previously only
reachable through the raw `map_sqlx_<db>_pool_opts` escape hatch.

- `test_before_acquire_if_idle_for(Duration)`: DB-agnostic shorthand that
  pings a pooled connection before handing it out only once it has been idle
  for at least the given duration, instead of on every acquire. Sets
  `test_before_acquire(false)`.
- `map_sqlx_{postgres,mysql,sqlite}_before_acquire`: per-backend passthrough of
  SQLx's `before_acquire` callback.
- `get_test_before_acquire` / `get_test_before_acquire_if_idle_for` getters.

Because SQLx's single `before_acquire` slot replaces rather than composes and
has no getter, a generic `apply_before_acquire` helper composes the idle-ping
shorthand with any user-provided callback into one closure (idle-ping first,
then the user callback). The helper returns the pool options untouched when
neither option is set, so existing behavior is unchanged.

Purely additive: no existing signature, default, or behavior is changed.