[tracking] issue for Block Elements

[cakekindel]

What this is

One of the next big steps for this library will be implementing Block Elements so consumers
can enable interactivity with their slack apps - if you're interested in helping contribute to this library,
start here or with the tracking issue for Composition Objects (#63)

How to contribute

• If a Child Issue doesn't have an issue associated, create one like this (Button block element #62)
  • if you can, use the issue to workshop the structure and API before diving in!
• If a Child Issue exists, and is not yet implemented or spoken for:
  • comment on the issue that you would like to work on it (that way nobody else accidentally does)
  • fork this repo
  • implement the feature on your fork
  • open a pull request closing the child issue.
  • for some closer guidance, feel free to reach out, or check out the commit history on this PR. It is a solid example of my personal process.

Must-haves for PRs

• A corresponding JSON smoke test
• Copied + formatted documentation from the [Block Element]s Slack API Docs
• (If applicable) Runtime validation that matches any length limits or similar requirement
• (If applicable) Runtime validation tests that exercise each model requirement

Nice-to-haves for PRs

• A method-based API that abstracts the fields away from library users, meaning:
  • A constructor with all required fields as parameters
    • e.g. fn from_field_a_and_field_b(field_a: A, field_b: B) -> Self
  • Setter methods for all optional fields
    • e.g. fn with_field_c(mut self, field_c: C) -> Self

Child Issues

• Button (Button block element #62)
• Checkboxes (Block Element: Checkboxes #88)
• Date Picker (Block Element: Date Picker #89)
• Image (Block Element: Image #90)
• Overflow Menu (Block Element: Overflow Menu #96)
• Plain-text input (Block Element: Plaintext Input #97)
• Radio button group (Block Element: Radio Buttons #98)
• Multi-select Menu
  • Static (Block Element: Multi-Select (static) #91)
  • External (Block Element: Multi-Select (External) #92)
  • Users (Block Element: Multi-Select (Users) #93)
  • Conversations (Block Element: Multi-Select (Conversations) #94)
  • Channels (Block Element: Multi-Select (Channels) #95)
• Select Menu
  • Static (Block Element: Select Menu (Static) #99)
  • External (Block Element: Select Menu (External) #100)
  • Users (Block Element: Select Menu (Users) #101)
  • Conversations (Block Element: Select Menu (Conversations) #102)
  • Channels (Block Element: Select Menu (Channels) #103)