Allow updating custom header values of webhook subscriptions

[pdecat]

This PR aims to allow updating custom header values of webhook subscriptions (AKA webhooks v3).

Currently, changes to the value field of these are completely ignored because:

• the PagerDuty API returns redacted values for those (-- redacted --),
• the provider stores those -- redacted -- values on read,
• the provider ignores all changes when the read value is redacted,
• rendering the ForceNew; true setting on the value field useless,

To fix this situation, my plan is to:

1. convert the custom_header field from schema.TypeList to schema.TypeSet, and let terraform compute a hash from the name and value for each header item,
2. and stop storing the redacted value read from the API (there's no point in storing the redacted value anyway).
3. We may add a check to store the value read if it differs from -- redacted -- (we never know, maybe the API will stop redacting all header values altogether at some point).

Alternatives considered:

• Storing a hash of the headers name/value pairs in some field, and mark that field as ForceNew; true.
• Request through PagerDuty support that the API behavior is modified to return a hash of the value instead of a static -- redacted -- value. Using a well-known and documented hash function would allow clients of the API to detect drift, and fix it.

That last option would be the best IMO, but I figured it will take time to implement on the PagerDuty API side,
and we need a solution in the short term to at least be able to update values.

[pdecat]

Right now, I've only added a test case to demonstrate custom header values cannot be updated:

# make testacc TEST=./pagerduty/ TESTARGS='-run=TestAccPagerDutyWebhookSubscription'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./pagerduty/ -v -run=TestAccPagerDutyWebhookSubscription -timeout 120m
=== RUN   TestAccPagerDutyWebhookSubscription_import
--- PASS: TestAccPagerDutyWebhookSubscription_import (13.63s)
=== RUN   TestAccPagerDutyWebhookSubscription_Basic
    resource_pagerduty_webhook_subscription_test.go:55: Step 2/2 error: Expected a non-empty plan, but got an empty plan
--- FAIL: TestAccPagerDutyWebhookSubscription_Basic (16.46s)
FAIL
FAIL    github.com/terraform-providers/terraform-provider-pagerduty/pagerduty   30.102s
FAIL
make: *** [GNUmakefile:17: testacc] Error 1

Other commits to fix the situation are coming...

[pdecat]

Hi @stmcallister @imjaroiswebdev, while I'm looking at implementing this change, WDYT about the underlying issue?

[pdecat]

Hi, gentle ping after the holiday season 🙏

[pdecat]

Hi @stmcallister @imjaroiswebdev, gentle ping six months after 🙂