support ldap userfilter attribute

[FalcoSuessgott]

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Relates OR Closes #0000

Release note for CHANGELOG:

IMPROVEMENTS: 

resource/vault_ldap_auth_backend - support "userfilter" attribute

Output from acceptance testing:

$ VAULT_TOKEN="root" VAULT_SKIP_VERIFY=true VAULT_ADDR="http://127.0.0.1:8200" make testacc TESTARGS='-run=LDAPAuthBackend'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test -v -run=LDAPAuthBackend -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]
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/helper    [no test files]
?       github.com/hashicorp/terraform-provider-vault/schema    [no test files]
?       github.com/hashicorp/terraform-provider-vault/testutil  [no test files]
testing: warning: no tests to run
PASS
ok      github.com/hashicorp/terraform-provider-vault/util      (cached) [no tests to run]
=== RUN   TestLDAPAuthBackendGroup_import
--- PASS: TestLDAPAuthBackendGroup_import (1.15s)
=== RUN   TestLDAPAuthBackendGroup_basic
--- PASS: TestLDAPAuthBackendGroup_basic (1.00s)
=== RUN   TestLDAPAuthBackend_import
--- PASS: TestLDAPAuthBackend_import (1.23s)
=== RUN   TestLDAPAuthBackend_basic
--- PASS: TestLDAPAuthBackend_basic (3.94s)
=== RUN   TestLDAPAuthBackend_tls
--- PASS: TestLDAPAuthBackend_tls (4.13s)
=== RUN   TestLDAPAuthBackendUser_import
--- PASS: TestLDAPAuthBackendUser_import (1.13s)
=== RUN   TestLDAPAuthBackendUser_basic
--- PASS: TestLDAPAuthBackendUser_basic (0.87s)
=== RUN   TestLDAPAuthBackendUser_noGroups
--- PASS: TestLDAPAuthBackendUser_noGroups (0.99s)
=== RUN   TestLDAPAuthBackendUser_oneGroup
--- PASS: TestLDAPAuthBackendUser_oneGroup (0.87s)
PASS
ok      github.com/hashicorp/terraform-provider-vault/vault     15.330s
...

[vinay-gopalan]

LGTM! Thanks for submitting this PR and your contribution to HashiCorp!