feat: Credential Injection for ext_authz and ext_proc calls

Title: Add Credential Injection compatible with ext_authz and ext_proc gRPC and HTTP calls

Description:

I have need for a feature facilitating workload authentication on external calls by the envoy.filters.http.ext_authz and envoy.filters.http.ext_proc filters. The Credential Injector filter almost works for this use case, but the secrets are only injected to the routed request, not the side-processing requests.

I don't know if this is best implemented as an enhancement to the ext_* filters to access SDS, or to somehow make secrets available to the existing API.