Added metadata field to VectorEnv and VectorWrapper. Implements #1005.

[TimSchneider42]

Description

Added metadata field to VectorEnv and VectorWrapper.

Fixes #1005

Type of change

• Documentation only change (no code changed)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

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]

Thanks for the PR, could you add some tests quickly?

[Kallinteris-Andreas]

Also please add something to the documentation About the metadata member variable

[pseudo-rnd-thoughts]

Thanks for the changes