sgID provider

**Is your feature request related to a problem? Please describe.** I would like a sgID provider. [sgID](https://id.gov.sg) is a OIDC provider for SingPass, part of the Singapore National Digital Identity suite of products. **Describe the solution you'd like** Would the team be open to a provider in this repository? My main concern is that the sgID platform is only open to Singaporeans. I have a rough implementation out and would be happy to open a PR for it. **Describe alternatives you've considered** If the team would prefer not to have such a provider due to it's more closed nature, no worries - please feel free to close this issue! **Additional context** N/A