feat grpc: ability to provide SSL certificate for GRPC client

[DmitriyH]

Currently, there is the only option to provide SSL certificate for GRPC client: from file.
It makes impossible to use a certificate received from an external secret data provider without saving it to disk, while saving private key to disk could potentially lead to a security issue.

It is suggested to declare an abstract component ugrpc::client::ExternalCredentialsProvider, which is used by ugrpc::client::ClientFactoryComponent. If ugrpc::client::ExternalCredentialsProvider returns a certificate, then SSL auth is enabled and certificate file path specified for factory config is ignored.

A minimal example:

// .hpp/.ccp

#include <userver/ugrpc/client/external_credentials_provider.hpp>

class GrpcClientCredentialsProvider final : public userver::ugrpc::client::ExternalCredentialsProvider {
 public:
  GrpcClientCredentialsProvider(const userver::components::ComponentConfig& config,
                                const userver::components::ComponentContext& context);

  std::optional<grpc::SslCredentialsOptions> GetSslCredentialsOptions() override {
    grpc::SslCredentialsOptions options;
    options.pem_cert_chain = ...;
    options.pem_private_key = ...;
    options.pem_root_certs = ...;
    return options;
  }
  
  ...
};

// static_config.yaml

    components:
        ...

        external-grpc-client-credentials-provider:

        ...