Simplify handling of Expand and ExtraParams

[ob-stripe]

r? @remi-stripe

Simplify handling of Expand and ExtraParams in BaseOptions:

• Expand is now annotated with [JsonProperty("expand")]
• ExtraParams is now annotated with [JsonExtensionData]

JsonExtensionData is a special attribute in Newtonsoft.Json that instructs the JSON serializer to write the values in the collection during serialization and to deserialize values with no matching class member during deserialization.

I added a few lines of code to FormEncoder to do something similar in our own serialization process. I also had to change the type of ExtraParams from Dictionary<string, string> to Dictionary<string, object>, but I think that's better anyway, as it lets uses use non-string extra params (e.g. a list or a hash).

Also, Expand is now null by default instead of an empty list, because otherwise it would be serialized as expand= when the list is empty. Using AddExpand() will automatically create the list if necessary.

[remi-stripe]

This looks great to me. There are some things I don't fully grasp so re-assign to @brandur-stripe to double check but feel free to self-approve and merge!

[brandur-stripe]

Seems way cleaner to me!

LGTM.

[ob-stripe]

Thanks both!