Add a new abstraction that represents a source for configuration
properties. The new source is similar to the `Environment` abstraction
provided by Spring Framework but follows a stricter set of rules.

The `ConfigurationPropertySource` provides a uniform view onto a source
and will help to move responsibility for accessing properties in a
"relaxed" way away from the caller.

The `ConfigurationPropertyName` class enforces strict naming rules
that callers must follow when accessing properties. Configuration
names are lowercase, dot separated and may contain dashes. In
addition "indexed" name elements may be defined by using square
brackets.

Mapping is provided to existing Spring PropertySources implemented with
the relaxed rules that users are used to. For example the configuration
property `server.local-host` can still be written in a property files
as `server.localHost` or in an environment variable as SERVER_LOCALHOST.

Closes gh-4910