chore: remote config client

[crleona]

Summary

Remote Configuration Spec
Configs are indefinitely cached locally by config key.
In general, newly fetched configs > cached configs > locally configured options
Plugins can be added at any time and still need to fetch updated remote configs.
We will use time based coalescing to combine requests - any remote config requests
within a window will all be sent within the same http request.
Browser clients currently wait for remote configuration before starting, and will
either need to convert to this model or continue to maintain a separate approach.
We may still need to preflight some keys (ie, request them outisde of this interface)
to enable some packaging like the unified snippet.

public class RemoteConfigClient {

    public typealias RemoteConfig = [String : Any]

    public typealias RemoteConfigCallback = (ConfigKey, RemoteConfig) -> Void

    /*
     * Synchronously returns a config from cache. Useful during setup
     */
    public func cachedConfig(key: ConfigKey) -> RemoteConfig?

    /*
     * Runs the callback every time the given key is updated, optionally doing an additional callback with the cached
     * config if one exists.
     * Useful when requesting a namespace ("sessionReplay"), then listening for changes to specific keys ("sessionReplay.privacyConfig")
     */
    public func listen(key: ConfigKey, returnCached: Bool = true, callback: @escaping RemoteConfigCallback)

    /*
     * Requests updates for the given key, and optionally adds a listener.
     */
    public func requestConfig(key: ConfigKey, returnCached: Bool = true, callback: RemoteConfigCallback? = nil)

    /*
    Refreshes all requested configs NOW.
    Configs should be stored in cache before any listeners are called (ie, listen(key) { config === cachedConfig(key) })
    */
    public func updateConfigs()
}

Simple Usage:

func requestRemoteConfigs() {}
    remoteConfigClient.register("sessionReplay.privacyConfig") { _, config in
        setting1 = config["setting1"]
        setting2 = config["setting2"]
    }
}

More advanced usage:

init() {

    if let cachedConfig = remoteConfigClient.cachedConfig(key: "sessionReplay.privacyConfig") {
        setting1 = config["setting1"]
        setting2 = config["setting2"]
    }

    if let cachedConfig = remoteConfigClient.cachedConfig(key: "sessionReplay.samplingConfig") {
        setting3 = config["setting3"]
        setting4 = config["setting4"]
    }

    remoteConfigClient.listen(key: "sessionReplay.privacyConfig", returnCached: false) { _, config in
        setting1 = config["setting1"]
        setting2 = config["setting2"]
    }

    remoteConfigClient.listen(key: "sessionReplay.samplingConfig", returnCached: false) { _, config in
        setting3 = config["setting3"]
        setting4 = config["setting4"]
    }

    remoteConfigClient.register(key: "sessionReplay")
}

Checklist

• Does your PR title have the correct title format?
• Does your PR have a breaking change?: no

[sojingle]

LGTM