Support for per-share bearer token configs

[atgabe]

At the moment authorization is everything or nothing, with a single bearer token providing access to the entire server. This PR expands authorization to support per-share bearer token configs.

• changes the authorization.bearerToken property to authorization.universalBearerToken, and adds an authorization.shares array to configure additional tokens unique to a share
• adds an AuthManager trait and two implementations (BearerTokenAuth and NoAuth) to handle authorization
• expands unit tests to cover per-share authorization cases

There are certainly other approaches to this feature, so let me know if an alternative approach is deemed preferable.

[zhuansunxt]

An alternative approach on this is to introduce Recipient as a first class concept, each with its own token and support fine-grained Share <-> Recipient access control. @atgabe Do you think that approach satisfy your needs?

[atgabe]

@zhuansunxt yup, that approach would work for our use case and sounds like it more closely mirrors the Databricks strategy. Is that approach in development elsewhere or a change you'd like to see on this PR?

[zhuansunxt]

@atgabe It would be great to submit an issue and use it to describe the general approach (like an RFC) so we can agree on design decisions.