add field to global config for setting the access log format to text or json

[GregHanson]

Corresponding envoy PR: envoyproxy/envoy#4693

In istio deployments without mixer, logs must be in json format so they can be parsed by a scraper

Would make sense to have the new access_log_format field by the access_log_file but not sure if it's worth introducing the potential breaking change - feedback welcome

[istio-testing]

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[rshriram]

move this next to access_log_path, retain numbering.. Have a comment on top of this entire proto, with something like Next available field number: 25