Improve config parsing

[rafd]

braid.base.api/register-config-var! allows modules to declare optional or required config vars (read from mount-args).

https://github.com/braidchat/braid/blob/master/src/braid/base/api.cljc#L149

Ex. braid.embeds-map does: (base/register-config-var! :google-maps-api-key)

However:

• there is no way to distinguish optional and required config vars
• there is no way to know what config vars could be set
• there is no way to know what the values of those config vars should be
• if incorrect config vars are passed in, braid happily accepts them and subtle issues happen later

Part 1:

• have register-config-var! take a second parameter, which is a malli schema ex. (base/register-config-var! :google-maps-api-key [:re #"some-regex-for-google-api-keys"])
• (may need to have a 3rd arg for optional vs required)
  • when 'reading' the config values (currently, here), check the values against the registered schema
  • change all uses of register-config-var! throughout Braid to use the 2-arg form

Part 2:

• provide a function in braid.dev.core to list the expected config

Below is some related code I pulled from another project:

(defn parse [config schema]
  (if (malli/validate (malli.util/closed-schema schema) config)
    config
    (let [cause (malli/explain (malli.util/closed-schema schema) config)]
      (throw
       (ex-info (str "Config invalid\n"
                     (with-out-str
                       (pprint/pprint (malli.error/humanize cause)))) cause)))))