gRPC endpoint for ListClients

[MrFoxTrot]

Preflight Checklist

• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for an issue that matches the one I want to file, without success.

Problem Description

Api already have List endpoints for passwords and refresh tokens.
When someone install dex with storage backend (for example sql) and decides to add clients via gRPC there is no way to list all existing clients

Proposed Solution

Add gRPC endpoint for ListClients

Alternatives Considered

No response

Additional Information

No response

[eLLIkin]

+1

[ifurs]

It would be great to have that feature too for my automation tool https://github.com/ifurs/dexus-vault