Support out-of-tree providers via gRPC interface

[guilhem]

Propose a standardized interface (e.g., gRPC) to allow implementing out-of-tree (external) providers in secretspec.

Use case: secret stores that are internal, private or closed source, enabled via an independent plugin (not part of upstream/open source).

Example scenarios:

• secretspec contacts an external provider via a gRPC or similar interface
• This enables organizations to add secret providers without exposing code
• Pattern similar to secret management integrations like Vault, 1Password, SOPS, etc.

This would make it easier to integrate proprietary or internal secrets sources without patching secretspec directly.

Thanks!

[domenkozar]

Could be a provider called grpc that has a spec written how it works. Happy to merge that :)

[domenkozar]

Could also be something more lightweight like Secretspec Server Protocol using JSONRPC