add datetimepicker component

[hussachai]

This is a new component that Slack supports Datetimepicker where date and time components are in the same row.
This new component takes number which I think it should be int64 for golang because Go's epoch API returns int64 data type.
Since datetimepicker takes two columns, it's not supported in Accessory. So, I don't include it as part of Section serialization/deserialization.

PS. Sorry I deleted and restored a couple times. First didn't realize that I pushed the upstream, then I changed my mind because I tested it and it's working as expected.

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[Houndie]

Thank you for doing this! Saved me some time for doing it myself <3

[kanata2]

[Question] Is it correct to say that this is because datetimepicker does not work with Section?

[hussachai]

I believe so, and the builder complains about this https://app.slack.com/block-kit-builder/T3E1TGDM3#%7B%22blocks%22:%5B%7B%22type%22:%22section%22,%22text%22:%7B%22type%22:%22mrkdwn%22,%22text%22:%22Pick%20a%20date%20for%20the%20deadline.%22%7D,%22accessory%22:%7B%22type%22:%22datetimepicker%22,%22action_id%22:%22datepicker-action%22%7D%7D%5D%7D

I assume that the space is not permitted for the layout. I hope Slack supports grid layout like Teams in future. Currently, everything seems to be limited to 2 columns at most.

[hussachai]

I might be wrong on this. Let me check one more time. Looks like it is supported
slackapi/java-slack-sdk@4176b91#diff-212bb028a409f158a94e9af3431b61e080b8bec0e72711ba99dc294e02fd5084L2866

[hussachai]

Working on it. Should be done soon. Thanks for asking this. The assumption needs to be tested.

[hussachai]

Hmm. I tested it out, and my assumption is right.

0 = {string} "[ERROR] failed to match all allowed schemas [json-pointer:/view]"
1 = {string} "[ERROR] unsupported type: datetimepicker [json-pointer:/view/blocks/8/accessory/type]"

datetimepicker won't work in Section. That kinda makes sense to me because I don't see any room to put text on it. It will go overflow.

[kanata2]

LGTM!