Add allowed_serial_numbers support

[axolotlgeoff]

This adds support for the allowed_serial_numbers setting in the vault_pki_secret_backend_role resource.

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Relates OR Closes #0000

Release note for CHANGELOG:

`resource/vault_pki_secret_backend_role`: Add `allowed_serial_numbers` option

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestPkiSecretBackendRole'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./...) -v -run=TestPkiSecretBackendRole -timeout 120m
?   	github.com/hashicorp/terraform-provider-vault	[no test files]
?   	github.com/hashicorp/terraform-provider-vault/cmd/coverage	[no test files]
?   	github.com/hashicorp/terraform-provider-vault/cmd/generate	[no test files]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-vault/codegen	(cached) [no tests to run]
?   	github.com/hashicorp/terraform-provider-vault/generated	[no test files]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-vault/generated/datasources/transform/decode	(cached) [no tests to run]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-vault/generated/datasources/transform/encode	(cached) [no tests to run]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-vault/generated/resources/transform/alphabet	(cached) [no tests to run]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-vault/generated/resources/transform/role	(cached) [no tests to run]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-vault/generated/resources/transform/template	(cached) [no tests to run]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-vault/generated/resources/transform/transformation	(cached) [no tests to run]
?   	github.com/hashicorp/terraform-provider-vault/schema	[no test files]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-vault/util	(cached) [no tests to run]
=== RUN   TestPkiSecretBackendRole_basic
--- PASS: TestPkiSecretBackendRole_basic (0.47s)
PASS
ok  	github.com/hashicorp/terraform-provider-vault/vault	0.483s

[hashicorp-cla]

All committers have signed the CLA.

[vinay-gopalan]

Thanks for submitting this PR and apologies for the delay! The PR is looking great!

[vinay-gopalan]

LGTM after merging commits, thanks for your contribution to HashiCorp!

[vinay-gopalan]

Since CI tests are failing due to this branch being on an older version of the codebase, the work has been taken over in a new PR forked from this branch. Closing this PR with #1441

Thanks once again for your contribution to HashiCorp!