Add support for authentication token

[kolesnikovae]

Recently we added support for agent authentication to Pyroscope (0.10.0). It would be nice to have support for this in the rust spy package.

The documentation can be found here: pyroscope-io/docs#43 (will be published soon!), the crux is that pyroscope server expects bearer authentication token in the Authorization header. For example:

printf "foo;bar 100\n foo;baz 200" | curl --fail \
-X POST \
-H "Authorization: Bearer ${PYROSCOPE_API_KEY}" \
--data-binary @- \
'http://localhost:4040/ingest?name=curl-test-app&from=1615709120&until=1615709130'

Existing integrations already support specifying an auth token via the configuration:

• Go: https://github.com/pyroscope-io/client/blob/11ad71f8e03fa690480f662e73998d920ee18ddc/pyroscope/remote.go#L162-L164
• Python: https://github.com/pyroscope-io/pyroscope-python/blob/11e524cf8fc9f50278ce57a104b50083e7874973/pyroscope/__init__.py#L14
• Ruby: https://github.com/pyroscope-io/pyroscope-ruby/blob/b04d19248345a1bae749969172859179358b8027/lib/pyroscope.rb#L14
• Java: https://github.com/pyroscope-io/pyroscope-java/blob/66cc8f7476bcc0a87dba7214103e21d7641ec5f2/agent/src/main/java/io/pyroscope/javaagent/Uploader.java#L103