Add documents for authentication setups

[crazyzhou]

Problem description Pravega has an open source implementation of the Keycloak client authentication, see https://github.com/pravega/pravega-keycloak. It's better to have a document to show how we set the configurations to talk to Keycloak-secured Pravega correctly. It could be either a new authentication.md document or enhance the sections in each API. Prefer the latter.

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