Make MetaDataHandle::get throw in case the value is not available and add more overloads

[tmadlener]

BEGINRELEASENOTES

• Make MetaDataHandle::get throw an exception in case the value for the handle is not (yet) available.
• Add MetaDataHandle::get(T defaultValue) overload to get a default value and no exception in case the value is not (yet) available.
• Add MetaDataHandle::get_optional() to retrieve an optional that is engaged and holds the value if the handle already has a value available.
  • The optional is returned directly from the underlying Frame in case that is available (see AIDASoft/podio#580) or is constructed in place in case it is not yet available.

ENDRELEASENOTES

~I have for now opted to let the underlying change in podio shine through here. I think having an optional value for MetaDataHandle::get offers the same benefits as having it on the Frame in the first place, as otherwise there is no way of distinguishing between empty and unset values.~

[tmadlener]

It looks like this has a rather small impact on downstream code, so I would be in favor of having this as a breaking change, i.e. the std::optional appearing in the return type.

[jmcarcell]

I'm fine with that, k4FWCore doesn't build anyway on the latest release, so not much is going to change

[tmadlener]

Now I have to introduce pre processor checks here and check that it works for podio @0.99 as well as once #580 is merged. I will be coming back to this a bit later (i.e. probably tomorrow).

[tmadlener]

Anything still missing from here, or can we merge this?

[tmadlener]

Merging this, since this is not transparent and we need to unblock AIDASoft/podio#580