Unified Way of Serializing Spaces and Samples from Spaces

Currently `gym.Spaces` as well as the sample drawn from them are serialized to JSON in different ways producing duplicated code and incompatible formats at a different places of the program.

Known serialization code locations:
* `dacbench.abstract_benchmark.AbstractBenchmark.load_config(...)`
* `dacbench.abstract_env.AbstractEnv.__init__(...)`
* `dacbench.container.container_utils.Encoder` and `dacbench.container.container_utils.Decoder`

Related to #91 and #80

Read the serialization paths in dacbench.abstract_benchmark.AbstractBenchmark.load_config(...), dacbench.abstract_env.AbstractEnv.__init__(...), and dacbench.container.container_utils.Encoder and Decoder. Compare how Spaces and their samples are encoded and decoded at each location. Done means these paths use one compatible JSON serialization format without duplicated, conflicting handling.

Written by the indexing model from the issue text.