[1.x] Added function to see when the trial is ending

[yoeriboven]

PR adds a trialEndsAt function to the Billable trait.

This allows you to write code like this:

@if ($user->onTrial())
    Your trial ends at {{ $user->trialEndsAt() }}.
@endif

Without this you'd first have to check whether the trial is generic or tied to a subscription.

It also lets you pass in a subscription name as the parameter ($user->trialEndsAt('main')).

[taylorotwell]

Tests failing?

[driesvints]

@taylorotwell all tests for prs on this repo will fail because OP's don't have their env variables set for github actions.

[driesvints]

I always make sure tests pass before releasing. This PR shouldn't break anything afaik.