Set user-agent when interacting with CCloud

[jlrobins]

Would be good for sidecar to interact with CCloud using a definite APIF team-approved User-Agent value that they will know how to aggregate.

[rhauch]

The Confluent for VS Code extension should include the following User-Agent header value in CCloud requests:

Confluent-for-VSCode/v0.1.2 (https://confluent.io; support@confluent.io) sidecar/v0.10.20 (linux/amd64)

or

Confluent-for-VSCode/v${EXTENSION_VERSION} (https://confluent.io; support@confluent.io) sidecar/v${SIDECAR_VERSION} (${PLATFORM}/${ARCH})

where

• ${EXTENSION_VERSION} is the major, minor and patch version of the Confluent for VS Code extension
• ${SIDECAR_VERSION} is the major, minor and patch version of the Sidecar
• ${PLATFORM} is the lowercase OS type/platform (e.g., linux, windows, mac, etc.)
• ${ARCH} is the architecture (e.g., amd64, x86) from JVM's os.arch

Changes to CCloud to recognize this agent are under way and should be in production in a few weeks, though those changes are not required and should not dictate the timing of this work.