(Describe your issue and goal here)

Hi - im not sure if this is a bug or just unexpected behavior. im working with the blocks library in the python SDK and the basic_components.Option objects that are used in RadioButtonElement, StaticSelectElement, etc require "plain_text" types for their text fields, but if you use the basic_components.Option(text="some random text", value="True") the text defaults to mrkdwn, which will then give an error about not matching schemas.

Reproducible in:

here is the json dump of what gets generated by the block kit sdk

>>> json.dumps(basic_components.Option(text="text0",value="text0").to_dict())
>>> {"text": {"text": "text0", "type": "mrkdwn"}, "value": "text0"}
but the default to mrkdwn for option text types does not work in certain block elements..
this is an example of a block that does not render bc the text types default to markdown

{
	"blocks": [
		{
			"type": "input",
			"element": {
				"type": "multi_static_select",
				"options": [
					{
						"text": {
							"text": "text0",
							"type": "mrkdwn"
						},
						"value": "text-0"
					},
					{
						"text": {
							"text": "text1",
							"type": "mrkdwn"
						},
						"value": "text-1"
					},
					{
						"text": {
							"text": "text2",
							"type": "mrkdwn"
						},
						"value": "text-2"
					}
				]
			},
			"label": {
				"emoji": true,
				"text": "Label 2 With Markdown Text in Input Block",
				"type": "plain_text"
			}
		}
	]
}

and then an example of an identical block with the only difference being the text type is now plain_text

{"blocks":[
		{
			"type": "input",
			"element": {
				"type": "multi_static_select",
				"placeholder": {
					"type": "plain_text",
					"text": "Select an item",
					"emoji": true
				},
				"options": [
					{
						"text": {
							"type": "plain_text",
							"text": "*text0*",
							"emoji": true
						},
						"value": "value-0"
					},
					{
						"text": {
							"type": "plain_text",
							"text": "*text1*",
							"emoji": true
						},
						"value": "value-1"
					},
					{
						"text": {
							"type": "plain_text",
							"text": "*text2*",
							"emoji": true
						},
						"value": "value-2"
					}
				],
				"action_id": "static_select-action"
			},
			"label": {
				"type": "plain_text",
				"text": "Label 1 With Plain Text in Input Block",
				"emoji": true
			}
		}]}

I'm curious why the default text_type for Option is mrkdwn if it is incompatible with some of the element types? i see this point in the code, but im curious if maybe theres a way to give an error message that lets developers know when this will cause issues with other interactive components.

The slack_bolt version

slack-bolt==1.6.1
slack-sdk==3.7.0

Python runtime version

Python 3.6.8 :: Anaconda, Inc.

OS info

ProductName:	Mac OS X
ProductVersion:	10.15.7

Steps to reproduce:

import json
from slack_sdk.models.blocks import block_elements, blocks, basic_components

json.dumps(basic_components.Option(text="text0",value="text0").to_dict())

>>> {"text": {"text": "text0", "type": "mrkdwn"}, "value": "text0"}

Expected result:

i expect that in using the basic_components.Option with a StaticSelectElement or StaticMultiSelectElement the resulting block would be defined and render appropriately.
within the Slack Bot Kit Builder

or expect an error message that tells you about the plain_text type requirement for some of the block_elements and which elements they are. from what ive seen its StaticSelectElement StaticMultiSelectElement

Actual result:

using the default types for text within a StaticSelectElement or StaticMultiSelectElement results in a schema violation:

Requirements

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