Include initial state manager for BipedalWalker

[arthur-plautz]

Description

The changes described in this PR contribute with the possibility of controlling the terrain generation for the BipedalWalkerV3.
Being able to retrieve and configure a specific terrain for this environment can be very useful for multiple use-cases, by allowing the user to have a greater visibility and control over the terrain.

The TerrainMetadata class was added to manage the read/write operations over random values generated to build the terrain, when calling the env.reset method. The code snippet below showcases the use of the this new functionality:

import gymnasium as gym

env = gym.make('BipedalWalker-v3')

# Use the default reset method / This will populate the `_terrain_metadata` property
env.reset()

# Retrieve the property directly through the `terrain_metadata` method
bipedal_env = env.unwrapped
metadata = bipedal_env.terrain_metadata()

# Use the `options` parameter to pass the terrain metadata and initialize the environment
options = dict(
    metadata=metadata
)
env.reset(options)

Further details on the motivation of this change are described in the referenced issue below.

Fixes # 892

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[pseudo-rnd-thoughts]

Interesting solution

Could you add tests and some general documentation on how to use this