Add scope separator for PlanningCenter provider

[roymckenzie]

This pull request introduces a minor update to the Provider class for Planning Center authentication. The main change is the addition of a scopeSeparator property, which defines how OAuth scopes are separated in authorization requests.

Authentication configuration:

• Added a protected $scopeSeparator property set to a space character in the Provider class to control how OAuth scopes are separated in requests.

This change was introduced because the Planning Center Authentication documentation notes that, 'The value should be a space-separated list of scopes'.

The default for BaseProvider is a comma.

Thank you!

[roymckenzie]

Thank you for the rapid merge @atymic!