[gpctl] Support batch-building workspace images

[csweichel]

This PR adds support for batch-building workspace images. This is particularly handy just before a deployment to pre-build the most used workspace images.

For example, the following will build the workspace images from all workspace instances started in the last 30 days.

mysql -N -B -u gitpod -p -h 127.0.0.1 gitpod \
	-e 'SELECT ws.baseImageNameResolved FROM d_b_workspace_instance wsi LEFT JOIN d_b_workspace ws ON ws.id = workspaceId WHERE wsi.creationTime > (NOW() - INTERVAL 30 DAY)' | \
	sort | \
	uniq | \
	gpctl imagebuilds build-batch

[aledbf]

LGTM

[roboquat]

LGTM label has been added.

Git tree hash: f679f77766a965c026af075954811d52bca3033d

[csweichel]

/approve no-issue

[aledbf]

/lgtm

[roboquat]

LGTM label has been added.

Git tree hash: 01d678a35bfeaae2564721a4cbb6dd8adb86753c

[roboquat]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, csweichel

Associated issue requirement bypassed by: csweichel

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