ChatGetPermalink endpoint: change message_ts type to string

[StyxUA]

Similar to #12, #22, #26 and #27.

[pyrech]

It looks like there is a lot of inconsistencies in how Slack describes/provides their timestamp 😆

We would be very glad if you could open a PR to fix this endpoint 😉 We have some documentation explaining how to update the SDK: https://github.com/jolicode/slack-php-api/blob/master/doc/updating-sdk.md#generating-a-new-patch

Do not hesitate to ask questions or for some help here 😉

[damienalexandre]

I think this issue is resolved already:

slack-php-api/generated/Endpoint/ChatGetPermalink.php

Line 67 in 4f8cd6a

$optionsResolver->setAllowedTypes('message_ts', ['string']);

Feel free to reopen if you still have any problem with the message_ts.