fix: Updated RichTextInputBlockElement InitialValue data type (#1320)

##### Pull Request Guidelines

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
- [x] 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.

Fix for [issue #1276](#1276)
Updated the datatype of RichTextInputBlockElement InitialValue from
string to *RichTextBlock

block_element.go

@@ -527,7 +527,7 @@ type RichTextInputBlockElement struct {
 	Type                 MessageElementType    `json:"type"`
 	ActionID             string                `json:"action_id,omitempty"`
 	Placeholder          *TextBlockObject      `json:"placeholder,omitempty"`
-	InitialValue         string                `json:"initial_value,omitempty"`
+	InitialValue         *RichTextBlock        `json:"initial_value,omitempty"`
 	DispatchActionConfig *DispatchActionConfig `json:"dispatch_action_config,omitempty"`
 	FocusOnLoad          bool                  `json:"focus_on_load,omitempty"`
 }