Support Grape 3 compatibility

[numbata]

This PR replaces the namespace_stackable access removed in Grape 3 with inheritable_setting usage so namespace aggregation still works, and raises the Grape dependency ceiling to < 4.0.

Fix #965

[dblock]

Looks good! Add to CHANGELOG, update README for versions, write some token_owner_resolver tests and we're good?

[numbata]

Absolutely! As usual.
But as no urgent rush here, I will do it tomorrow evening (by Stockholm time) and will look to the rest of the gems as well.

Probably CI Matrix needs to be updated also.

[dblock]

@numbata looks like legit CI failures

[numbata]

@dblock yes. Working on it. The issue is that helpers also should be taken for account during resolving.