allow JWT properties to be stored under non-string context key

[TheMacies]

What

• This PR changes the property key to be used in request context to interface{}
• Changes are backwards compatible, because string satisfies interface{}

Why

Storing context values with string keys is not a clean design. A conflict may arise if other package were to add a 'user' value to the context. A solution is to use predefined package's private context keys. The problem and way of creating such keys are both described here.

This PR enables that behaviour.

[sergiught]

We just released the v2.0.0-beta 🥳 !

You can start testing it by running go get github.com/auth0/go-jwt-middleware/v2@v2.0.0-beta.

In case of issues fetching the v2 you might want to try go clean --modcache first before doing go get.

I'm closing this pr as now this is part of v2, but feel free to reopen if needed.