Extract RUS7 Balance sheet table

[cmgosnell]

Overview

Working on #4897.

What problem does this address?

What did you change?

Documentation

Make sure to update relevant aspects of the documentation:

• Update the release notes: reference the PR and related issues.
• Update relevant Data Source jinja templates (see docs/data_sources/templates).
• Update relevant table or source description metadata (see src/metadata).
• Review and update any other aspects of the documentation that might be affected by this PR.

Testing

How did you make sure this worked? How can a reviewer verify this?

To-do list

• If updating analyses or data processing functions: make sure to update row count expectations in dbt tests.
• Run pixi run pre-commit-run to run linters and static code analysis checks.
• Run pixi run pytest-ci locally to ensure that the merge queue will accept your PR.
• Review the PR yourself and call out any questions or issues you have.
• For PRs that change the PUDL outputs significantly, run the full ETL locally and then run the data validations using dbt. If you can't run the ETL locally then run the build-deploy-pudl GitHub Action manually and ensure that it succeeds.

[aesharpe]

Does it make sense / do you want to have skiprows / skipheaders / init files or is that not necessary?

[cmgosnell]

the skiprows etc. isn't necessary for the csv extractors! (we've only ever seen csv's that just start on the first row thus far)