Title: expose generic secret to golang http filter

Description:
Among other things, Golang filter currently has access to filter state and dynamic metadata. Those are not ment to store secret.
Also, passing secret directly into the plugin config is not benefiting from envoy's SDS and other features around secrets. It also can leak secrets when dumping the configuration.

Proposed solution:

• declare the secrets accessible from go plugin in the go plugin configuration.
• introduce a SecretManager interface in the GO envoy SDK
• wire the go SecretManager with C++ SecretManager, filtering on the exposed secrets