This commit introduces the `DataRequestVariable` abstract base class and
its concrete implementation `CMIP6DataRequestVariable`. The new classes
use the `dataclass` decorator to automatically generate special methods
like `__init__` and `__repr__`.

The `DataRequestVariable` class outlines the necessary properties and
methods that any variable class should implement. The
`CMIP6DataRequestVariable` class is a concrete implementation for CMIP6
variables.

Additionally, this commit includes methods for constructing
`DataRequestVariable` instances from dictionaries and JSON files, as
well as converting instances to dictionary representations.

Unit tests for `CMIP6DataRequestVariable` have been added to ensure
correct functionality.