add 'expand' option to section block

[mrvovanness]

this allow to set 'expand' option as documented here: https://api.slack.com/reference/block-kit/blocks#section_fields

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These are recommendations for pull requests.
They are strictly guidelines to help manage expectations.

PR preparation

Run make pr-prep from the root of the repository to run formatting, linting and tests.

Should this be an issue instead

• is it a convenience method? (no new functionality, streamlines some use case)
• exposes a previously private type, const, method, etc.
• is it application specific (caching, retry logic, rate limiting, etc)
• is it performance related.

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• no tests, if you're adding to the API include at least a single test of the happy case.
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• in library cache for users. caches are use case specific.
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[MattDavisRV]

I included these changes in PR-1331 after opening it for the AI events being added. Seems to be the exact same code as this PR, if you want to consolidate them.