fix: Add required format field to rich text date blocks (#1317)

As per [block kit
docs](https://api.slack.com/reference/block-kit/blocks#date-element-type),
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.

block_rich_text.go

@@ -414,16 +414,22 @@ func NewRichTextSectionUserGroupElement(usergroupID string) *RichTextSectionUser
 type RichTextSectionDateElement struct {
 	Type      RichTextSectionElementType `json:"type"`
 	Timestamp JSONTime                   `json:"timestamp"`
+	Format    string                     `json:"format"`
+	URL       *string                    `json:"url,omitempty"`
+	Fallback  *string                    `json:"fallback,omitempty"`
 }
 
 func (r RichTextSectionDateElement) RichTextSectionElementType() RichTextSectionElementType {
 	return r.Type
 }
 
-func NewRichTextSectionDateElement(timestamp int64) *RichTextSectionDateElement {
+func NewRichTextSectionDateElement(timestamp int64, format string, url *string, fallback *string) *RichTextSectionDateElement {
 	return &RichTextSectionDateElement{
 		Type:      RTSEDate,
 		Timestamp: JSONTime(timestamp),
+		Format:    format,
+		URL:       url,
+		Fallback:  fallback,
 	}
 }
 

block_rich_text_test.go

@@ -167,13 +167,14 @@ func TestRichTextSection_UnmarshalJSON(t *testing.T) {
 		err      error
 	}{
 		{
-			[]byte(`{"elements":[{"type":"unknown","value":10},{"type":"text","text":"hi"},{"type":"date","timestamp":1636961629}]}`),
+			[]byte(`{"elements":[{"type":"unknown","value":10},{"type":"text","text":"hi"},{"type":"date","timestamp":1636961629,"format":"{date_short_pretty}"},{"type":"date","timestamp":1636961629,"format":"{date_short_pretty}","url":"https://example.com","fallback":"default"}]}`),
 			RichTextSection{
 				Type: RTESection,
 				Elements: []RichTextSectionElement{
 					&RichTextSectionUnknownElement{Type: RTSEUnknown, Raw: `{"type":"unknown","value":10}`},
 					&RichTextSectionTextElement{Type: RTSEText, Text: "hi"},
-					&RichTextSectionDateElement{Type: RTSEDate, Timestamp: JSONTime(1636961629)},
+					&RichTextSectionDateElement{Type: RTSEDate, Timestamp: JSONTime(1636961629), Format: "{date_short_pretty}"},
+					&RichTextSectionDateElement{Type: RTSEDate, Timestamp: JSONTime(1636961629), Format: "{date_short_pretty}", URL: strp("https://example.com"), Fallback: strp("default")},
 				},
 			},
 			nil,
@@ -361,3 +362,5 @@ func TestRichTextQuote_Marshal(t *testing.T) {
 		}
 	})
 }
+
+func strp(in string) *string { return &in }