Is your feature request related to a problem? Please describe.

When using an assumed role in AWS China, cert-manager uses the default STS endpoint of https://sts.amazonaws.com, but the correct one should be https://sts.<region-code>.amazonaws.com.cn (ref).

Describe the solution you'd like

Should determine the sts endpoint by region, maybe endpoints api?

Describe alternatives you've considered

Additional context

/kind feature