terraform: use kubectl_manifest

[tgeoghegan]

The kubernetes_manifest resource in provider hashicorp/kubernetes
has a known issue[1] where resources created in a manifest can't depend
on other resources that don't exist yet. To work around this, we instead
use gavinbunney/kubectl's kubectl_manifest resource, which does not
have this problem because it uses a different mechanism for planning.

[1] hashicorp/terraform-provider-kubernetes#1380

Resolves #1088

[codecov-commenter]

Codecov Report

Merging #1262 (cfe13c7) into main (048d8be) will not change coverage.
The diff coverage is n/a.

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@@           Coverage Diff           @@
##             main    #1262   +/-   ##
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  Coverage   58.36%   58.36%           
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  Files          38       38           
  Lines        7397     7397           
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  Hits         4317     4317           
  Misses       2972     2972           
  Partials      108      108           

Flag	Coverage Δ
facilitator_tests	60.56% <ø> (ø)
key_rotator_tests	57.58% <ø> (ø)
workflow_manager_tests	38.50% <ø> (ø)

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[tgeoghegan]

Pulling in another TF provider just to work around a bug in the Hashicorp provider isn't ideal, but the alternative would have been to introduce a multi-stage Terraform process so that we could guarantee that all Kubernetes namespaces already exist before we try to deploy a manifest, so I think this is better.