Add vault_config_ui_custom_message resource

[marcboudreau]

Description

This PR introduces a new resource named vault_config_ui_custom_message to the provider to allow managing the newly introduced UI Custom Messages in Vault v.1.16.0.

Checklist

• [x] Added CHANGELOG entry (only for user-facing changes)
• [x] Acceptance tests where run against all supported Vault Versions

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccConfigUICustomMessage'

==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test -run=TestAccConfigUICustomMessage -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]
?       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/codegen   0.658s [no tests to run]
?       github.com/hashicorp/terraform-provider-vault/internal/identity/group   [no test files]
?       github.com/hashicorp/terraform-provider-vault/internal/identity/mfa     [no test files]
?       github.com/hashicorp/terraform-provider-vault/internal/pki      [no test files]
?       github.com/hashicorp/terraform-provider-vault/internal/sync     [no test files]
?       github.com/hashicorp/terraform-provider-vault/schema    [no test files]
ok      github.com/hashicorp/terraform-provider-vault/internal/identity/entity  0.859s [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/internal/provider 2.097s [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/testutil  1.292s [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/util      1.446s [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/vault     3.706s

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[marcboudreau]

I've added the do-not-merge label for now, because this change should only be adopted once Vault v.1.16.0 is release.

[fairclothjm]

Clearing out the 4.0.0 milestone for now and we will do another release closer to Vault GA.