Add StripeContext object

[xavdid-stripe]

Why?

As the stripe-context header is evolving, we want to give better tools for creating and managing contexts. This PR adds a new class, StripeContext. It can be used anywhere the context option is supplied and gets serialized to a string when making requests. It's also found on the EventNotification.context property.

What?

• add StripeContext class
• allow it to be supplied for RequestOptions.context; serialize it into strings
• add tests

Changelog

• Add the StripeContext class
• ⚠️ Change EventNotification (formerly known as ThinEvent)'s context property from string to StripeContext

[xavdid-stripe]

closed in favor of #2057