access log: add JSON logging mode

[aa-stripe]

These are the code changes to add a JSON access logging mode. I am working on the docs/release notes, but wanted to get started on code review!

Description:

• Add a new config option under access_log called json_format. This is a single level dictionary that contains strings as keys, and envoy access log format specifiers (such as %PROTOCOL%) as values. The specifiers will be replaced with actual values at logging time. I call this dictionary the "format dictionary" (as opposed to "format string").
• You can specify only one of format (format string) or json_format (format dictionary). If neither are there, we fall back to the default string format.
• Add the correct plumbing inside the configuration parsing to handle this.
• Add a new access log formatter class that is instantiated with the format dictionary. It maintains the mapping of dictionary keys to loggers
• Create a new class called FormatterProvider, to distinguish things that actually extract the information from a request. The things that combine together a bunch of FormatterProviders are still called Formatters. This is primarily a semantic/naming difference, but imo these are two conceptually separate things. There is, however no API difference, and if people are truly opposed to this, I could just merge them back into one Formatter class. This also provides a better foundation for adding more log formats in the future.
• At present, only one specifier per key in the format dictionary is allowed. This is because the whole point of JSON logging is to make logs easily machine-parseable. If you can include multiple formats in the same field, then you'll be right back to parsing those manually
• At present, only top-level keys are allowed in the format dictionary. This is validated at config load time. In the future, we can expand this to have nested dictionaries.

Risk Level:
Low. It's an optional feature that has to be explicitly enabled.

Testing:
Unit testing for the actual formatter, and config load. Also manually tested using an example config file.

Docs:
Will have to add docs. Could someone please advise on how to preview the docs so that I can make sure they are good as I write them?

Release Notes:
Will have to add Release notes

Fixes #2692

[aa-stripe]

Working on getting this building again. The recent RequestInfo->StreamInfo change messed things up a little.

[alyssawilk]

Yeah, I think your branch may have some screwed up state. Feel free to either ping when it's ready for review, or if you need to close it out and start from scratch that's fine too.

[zuercher]

You can build the docs via ./docs/build.sh. I believe it will install the necessary python packages automatically. The docs are generated into generated/docs/.

[aa-stripe]

Yeah, I fixed the StreamInfo issue, but things got real screwed up when I did the signoff rebase. I'm trying to fix this now. Thanks @zuercher, I'll get that running locally

[aa-stripe]

Closing in favor of #4693