Make JsonReadContext non-final

[isalvadori]

After opening a discussion here about the need (or not) for the JsonReadContext to be final, it turned out there is no strong reason behind having it final.
Thus, I would like to propose to remove the final on that class.
This will allow easier customization of the context.

[maho7791]

I wanted to mention that we would need this also for the 3.x versions.
We currently move our EMF mapping for jackson3.

[cowtowncoder]

Yes, we merge forward changes in most cases, and do so here.

[maho7791]

This was faster than light!!!

[cowtowncoder]

@maho7791 fwtw this was due to lucky timing. It's not always this fast -- but sometimes that happens which is great :)