Add user agent version based on cargo package version

[russcam]

This commit adds a default user-agent header to the client of the form

elasticsearch-rs/{version}

where version is the cargo package version. This format matches the agreed format for official language clients.

Update the package versions for the client and api_generator to match that of the REST API spec version from which the client is generated. The client versioning will follow at least the major/minor of Elasticsearch, similar to other official clients.