Eclipse Hono 2.1.0

This minor release introduces the following new features

• The tracing configuration has been extended to now also support a rate-limiting sampler. See the Monitoring & Tracing guide for details.
• Add new metric for tracking the total number of Tenants in Hono.
• The Auth server component now provides an HTTP endpoint for retrieving the keys that applications should use for validating the signature of tokens issued by the Auth server. The keys are being transferred by means of a JSON Web Key set as defined by RFC 7517. The Device Registry and  Command Router components will use this endpoint to periodically download the keys if no key material has been configured explicitly.
• The handling of messages that have no payload and/or no content type has been harmonized across the protocol adapters.
• Adapters now uniformly accept messages from devices that have no payload if the device explicitly sets a non-empty content type on the message. Messages with a non-empty payload can be uploaded without specifying a content type. In the messages being forwarded to downstream consumers, the adapters will either use the devices's default content type, if defined, or otherwise fall back to `application/octet-stream` in such cases.
• A Linux x86_64 executable of the command line client has been added to the downloads page.