[5.7] Add relationship getters

[staudenmeir]

Some of the relationship properties don't yet have their own getters, so there's no (good) way to access them from the outside.

[antonkomarev]

Such changes should be targeted to master branch and not stable 5.7.
Could you provide any examples why do you need to have access to this properties?

[deleugpn]

I don't think there's enough cause for targeting master on these changes. Although I am curious as to the use-case for it.

[staudenmeir]

@antonkomarev Why the master branch? There are no breaking changes.

I ran into this while working on one of my packages. Users can create a relationship by chaining multiple of their existing relationships. To implement this, I have to access all the relationships' keys. At the moment, I'm using the Closure::bind() workaround to get the protected properties.

My package aside, I think those properties should be accessible. There might be other use cases.