Add vault_namespaces data source

[tmatilai]

Lists paths of all direct child namespaces.

Closes #995
Relates #1768

Output from acceptance testing

$ make testacc-ent TESTARGS='-run=TestAccDataSourceNamespace'
make testacc TF_ACC_ENTERPRISE=1
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test -run=TestAccDataSourceNamespaces -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   (cached) [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    (cached) [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/generated/datasources/transform/encode    (cached) [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/generated/resources/transform/alphabet    (cached) [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/generated/resources/transform/role        (cached) [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/generated/resources/transform/template    (cached) [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/generated/resources/transform/transformation      (cached) [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  (cached) [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]
ok      github.com/hashicorp/terraform-provider-vault/internal/provider (cached) [no tests to run]
?       github.com/hashicorp/terraform-provider-vault/schema    [no test files]
ok      github.com/hashicorp/terraform-provider-vault/testutil  (cached) [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/util      (cached) [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/vault     10.784s

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[tmatilai]

This example bit depends on #1768

[fairclothjm]

@tmatilai Thanks for the contribution!

I am also getting a build error on this PR

 $ make dev
==> Checking that code complies with gofmt requirements...
go build -o terraform-provider-vault
# github.com/hashicorp/terraform-provider-vault/vault
vault/data_source_namespaces.go:21:16: undefined: ReadContextWrapper
vault/data_source_namespaces.go:42:37: undefined: SysNamespaceRoot
make: *** [dev] Error 1