[WAF] add Data Masking rule resource

[chengxiangdong]

What this PR does / why we need it:

Support the management of the data masking rules of the policy.

• Create rules for the specified policy.
• Update the rules.
• Delete the existing rule.
• Support for importing existing rules from HUAWEI CLOUD to Terraform.

Which issue this PR fixes:
(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)
fixes #1280

Special notes for your reviewer:

• The field supports 4 parameters: header, params, form and cookie, which are not fully supplemented in the API documentation.

Release note:

1. Added resource('huaweicloud_waf_rule_data_masking') for manging the rules of policy. 
2. For complete use, it needs to be released together with WAF policy management.  And WAF policy management has been merged into master.

PR Checklist

• Tests added/passed.
• Documentation updated.
• Schema updated.

Acceptance Steps Performed

make testacc TEST='./huaweicloud/services/acceptance/waf' TESTARGS='-run TestAccWafRuleDataMasking_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud/services/acceptance/waf -v -run TestAccWafRuleDataMasking_basic -timeout 360m -parallel 4
=== RUN   TestAccWafRuleDataMasking_basic
=== PAUSE TestAccWafRuleDataMasking_basic
=== CONT  TestAccWafRuleDataMasking_basic
--- PASS: TestAccWafRuleDataMasking_basic (33.45s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/waf       33.540s