resource/github_organization_webhook: Avoid spurious diff

[radeksimko]

Similar to #133 the same bug appears in org webhook, which has pretty much the same schema, so I refactored some of the functionality to reuse it.

Here's a test failing prior to the patch:

TF_ACC=1 go test ./github -v -run=TestAccGithubOrganizationWebhook_ -timeout 120m
=== RUN   TestAccGithubOrganizationWebhook_basic
--- PASS: TestAccGithubOrganizationWebhook_basic (3.22s)
=== RUN   TestAccGithubOrganizationWebhook_secret
--- FAIL: TestAccGithubOrganizationWebhook_secret (1.38s)
	testing.go:518: Step 0 error: After applying this step, the plan was not empty:

		DIFF:

		UPDATE: github_organization_webhook.foo
		  configuration.secret: "********" => "VerySecret"

		STATE:

		github_organization_webhook.foo:
		  ID = 44243925
		  provider = provider.github
		  active = true
		  configuration.% = 4
		  configuration.content_type = json
		  configuration.insecure_ssl = 0
		  configuration.secret = ********
		  configuration.url = https://www.terraform.io/webhook
		  events.# = 1
		  events.2342231791 = pull_request
		  name = web
		  url = https://api.github.com/orgs/terraformtesting/hooks/44243925
FAIL
FAIL	github.com/terraform-providers/terraform-provider-github/github	4.633s

After patch

TF_ACC=1 go test ./github -v -run=TestAccGithubOrganizationWebhook_ -timeout 120m
=== RUN   TestAccGithubOrganizationWebhook_basic
--- PASS: TestAccGithubOrganizationWebhook_basic (2.23s)
=== RUN   TestAccGithubOrganizationWebhook_secret
--- PASS: TestAccGithubOrganizationWebhook_secret (1.21s)
PASS

[appilon]

I can be persuaded to let it slide but it doesn't feel like a great place for the config func. Other than that looks good.

[appilon]

Don't love where this lives, maybe create a file named something like webhook_helpers.go

[radeksimko]

Ah, yeah, I had mixed feelings about that too, let me clean it up 🙂

[radeksimko]

@appilon Moved the schema into its own file, PTAL.