fix: Add required `format` field to rich text date blocks

[luke-josh]

As per block kit docs, the date element requires a format string to be included. Currently, submitting a block kit with this element results in a slack API error.

Also added the two optional fields url and fallback for posterity.

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.

API changes

Since API changes have to be maintained they undergo a more detailed review and are more likely to require changes.

• no tests, if you're adding to the API include at least a single test of the happy case.
• If you can accomplish your goal without changing the API, then do so.
• dependency changes. updates are okay. adding/removing need justification.

Examples of API changes that do not meet guidelines:

• in library cache for users. caches are use case specific.
• Convenience methods for Sending Messages, update, post, ephemeral, etc. consider opening an issue instead.