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.

[lorenzoaiello]

Apologies for the delay, and thank you for your contribution!

This pull request fixes rich text date blocks by including the format field as documented in the public block kit documentation. Additionally, it adds the ability to set the url and fallback fields which are also publicly documented.

Note: This does include breaking changes to the NewRichTextSectionDateElement function.