Add `vault_namespace` data source

[tmatilai]

Add vault_namespace data source for fetching information of specified or the current namespace.

Closes #1748 Relates #1766 Relates #1756

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=TestAccDataSourceNamespace -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     10.998s

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[tmatilai]

@fairclothjm Thanks for the feedback and attention! I didn't want to use more time on keeping the PRs up-to-date before getting any signal that they could be accepted.

I have a bit busy week again, but will definitely try to rebase and revisit these soon. 🙂

[fairclothjm]

@tmatilai FYI, I opened a PR which will supersede this PR.

[tmatilai]

@fairclothjm Thank you so much! Sorry, I just haven't found the time and energy to clean these up. So many things going on... 😅