Added fields to the observation space.

[ppasupat]

Description

Added fields to the observation space.

• Fields are key-value pairs extracted from the utterance. For example, "Click on the OK button" → [("target", "OK")].
• Fields are represented as a sequence of key-value tuples.

Fixes #29.

Type of change

Please delete options that are not relevant.

• 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]

If you are happy with the testing then Im happy with the PR. You understand the project better than I do