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](https://www.envoyproxy.io/docs/envoy/latest/configuration/http/http_filters/ext_authz_filter) and [envoy.filters.http.ext_proc](https://www.envoyproxy.io/docs/envoy/latest/configuration/http/http_filters/ext_proc_filter.html) filters. The [Credential Injector](https://www.envoyproxy.io/docs/envoy/latest/configuration/http/http_filters/credential_injector_filter.html) 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.