jwt_auth_backend: fix oidc_client_secret storage

[coreypobrien]

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Closes #449

The oidc_client_secret is sensitive so it will not be in the response from Vault.

Our options are to a) always assume it must be updated (current) or b) always assume it matches what we last set it to (this change). This assumes it always matches what we last set it to so that HasChange isn't always true.

This does intentionally miss the edge case where the oidc_client_secret is updated without using Terraform. Since we cannot know the current state of oidc_client_secret, Terraform will show no changes are required even if the actual value in Vault does not match the current value in state.

Release note for CHANGELOG:

Store `oidc_client_secret` value correctly for `jwt_auth_backend` to fix bug where `oidc_client_secret` always appears to require an update. 

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccJWTAuthBackend_OIDC'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run=TestAccJWTAuthBackend_OIDC -timeout 120m
?   	github.com/terraform-providers/terraform-provider-vault	[no test files]
?   	github.com/terraform-providers/terraform-provider-vault/cmd/coverage	[no test files]
testing: warning: no tests to run
PASS
ok  	github.com/terraform-providers/terraform-provider-vault/util	(cached) [no tests to run]
=== RUN   TestAccJWTAuthBackend_OIDC
--- PASS: TestAccJWTAuthBackend_OIDC (0.84s)
PASS
ok  	github.com/terraform-providers/terraform-provider-vault/vault	1.186s

[coreypobrien]

That failure seems to be a flaky test right? Looks like it also failed on master a few builds ago: https://app.circleci.com/pipelines/github/terraform-providers/terraform-provider-vault/1067/workflows/0131be17-4682-4e77-939f-0ec24948d676/jobs/1063

[coreypobrien]

@catsby Is there a step I'm missing to get this assigned for review?

[catsby]

This looks great, thanks! Sorry for the delay