Move icuTokenizer flag into featureFlags object in config

[SiarheiFedartsou]

👋 I did some awesome work for the Pelias project and would love for everyone to have a look at it and provide feedback.

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Here's the reason for this change 🚀

There is a problem is that schema config is used in other repos as well, e.g. in https://github.com/pelias/dbclient and our config validation is very strict, i.e. it doesn't allow any extra fields (if unknown(true) is not called explicitly), so we basically cannot set icuTokenizer = true if the same config is used in other repos. So the idea (by @orangejulius) is to introduce separate featureFlags object in which we have less strict validation.

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Here's what actually got changed 👏

1. Added featureFlags to config schema (with unknown(true) in order to allow to use the same field in other repos for feature flags if needed)
2. Moved schema.icuTokenizer to featureFlags.icuTokenizer

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Here's how others can test the changes 👀

I updated tests...

Previous discussion pelias/dbclient#131