Natural Language

[lukesneeringer]

This PR is an amalgamation of:

• Natural Language: Auto-gen update. #3963 (which shuffles the auto-gen code around)
• Add AnalyzeEntitySentiment method to language v1. #3968 (which adds analyze_entity_sentiment as a method)
• Remove the natural language manual layer. #3970 (which removes the manual layer)
• Natural Language 0.29.0 #3972 (which bumps the version number)

[dhermes]

Rubber stamp LGTM

[lukesneeringer]

Release notes:

• Full removal of the deprecated surface using google.cloud.language.Client. (Remove the natural language manual layer. #3970)
• Addition of analyze_entity_sentiment (Add AnalyzeEntitySentiment method to language v1. #3968)