…t for image block

Fixes #1244

Closes #1247

This diff adds support for the "Timezone" parameter in the slack
timepicker block element.
See https://api.slack.com/reference/block-kit/block-elements#timepicker
for details.

By default this will be set to the empty string, equivalent to not set.
This can be overridden by the user.

#1447)

Fixes #1244

Closes #1247

Supporting slack_file in ImageBlockElement: 1438
- Add support for slack_file to ImageBlockElement
- update tests involving ImageBlockElement where ImageURL is set as a
function arg
- fix support for slack_file in ImageBlock
- update tests involving ImageBlock where ImageURL is set as a function
arg
- Update examples for both ImageBlock and ImageBlockElement

This does change the function signature of both NewImageBlockElement and
NewImageBlock. If you would instead prefer slack-file specific versions
of each of these functions so that the existing function signature can
remain the same, let me know.

This diff completes the rest of the implementation from #1448 , as it
was accidentally missed out of the first PR.
The first PR allows the user to provide a Timezone to the slack model in
blockkit, but this PR allows the application to read state updates from
slack (i.e. read a changed timezone value)

Closes #1450.

Closes #1450.