Add vault_audit_request_header resource

[geekofalltrades]

Supports the sys/config/auditing/request-headers APIs.

Closes #1465

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Closes #1465

Release note for CHANGELOG:

* **New Resource:** `vault_audit_request_header` ([#1610](https://github.com/terraform-providers/terraform-provider-vault/pull/1610)).

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAuditRequestHeader'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test -run=TestAuditRequestHeader -timeout 30m ./...
?       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]
ok      github.com/hashicorp/terraform-provider-vault/codegen   0.021s [no tests to run]
?       github.com/hashicorp/terraform-provider-vault/generated [no test files]
ok      github.com/hashicorp/terraform-provider-vault/generated/datasources/transform/decode    0.042s [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/generated/datasources/transform/encode    0.023s [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/generated/resources/transform/alphabet    0.019s [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/generated/resources/transform/role        0.030s [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/generated/resources/transform/template    0.024s [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/generated/resources/transform/transformation      0.024s [no tests to run]
?       github.com/hashicorp/terraform-provider-vault/helper    [no test files]
?       github.com/hashicorp/terraform-provider-vault/internal/consts   [no test files]
ok      github.com/hashicorp/terraform-provider-vault/internal/identity/entity  0.025s [no tests to run]
?       github.com/hashicorp/terraform-provider-vault/internal/identity/group   [no test files]
?       github.com/hashicorp/terraform-provider-vault/internal/pki      [no test files]
ok      github.com/hashicorp/terraform-provider-vault/internal/provider 0.026s [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/internal/semver   0.017s [no tests to run]
?       github.com/hashicorp/terraform-provider-vault/schema    [no test files]
ok      github.com/hashicorp/terraform-provider-vault/testutil  0.042s [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/util      0.019s [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/vault     2.105s

...

[hashicorp-cla]

All committers have signed the CLA.