Introduce ModelAccessor and ModelPointer API

[beikov]

An API that allows to point to an abstract attribute or a concrete element in an object. This is similar to JSONPointer which we want to support for serialization/deserialization.
The default serialized format is the JPAish dot-navigation syntax we use in blaze-persistence and also in the parser for blaze-expression paths.

The ModelPointer can be used to query and write values on actual domain objects via the ModelAccessor API. Note that there already is an AttributeAccessor Metadata API in blaze-expression and the idea is, that the ModelAccessor API is the successor that lives in blaze-domain.

In the end, I want to be able to do simple object graph extraction and altering with just blaze-domain. Some simple ModelAccessors for accessing a Map should also be provided. Note that the ModelAccessor is format dependent, so if we expose this somehow in the API, the format dependence should be encoded into that as well.