[rds] support prePaid charging mode for rds instance

[ShiChangkuo]

What this PR does / why we need it:
fixes #1066

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

Special notes for your reviewer:

Release note:

PR Checklist

• [√] Tests added/passed.
• [√] Documentation updated.
• [√] Schema updated.

Acceptance Steps Performed

$ make testacc TEST='./huaweicloud' TESTARGS='-run TestAccRdsInstanceV3_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run TestAccRdsInstanceV3_basic -timeout 360m -parallel 4
=== RUN   TestAccRdsInstanceV3_basic
=== PAUSE TestAccRdsInstanceV3_basic
=== CONT  TestAccRdsInstanceV3_basic
--- PASS: TestAccRdsInstanceV3_basic (1015.96s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       1016.011s

$ make testacc TEST='./huaweicloud' TESTARGS='-run TestAccRdsReadReplicaInstance_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run TestAccRdsReadReplicaInstance_basic -timeout 360m -parallel 4
=== RUN   TestAccRdsReadReplicaInstance_basic
=== PAUSE TestAccRdsReadReplicaInstance_basic
=== CONT  TestAccRdsReadReplicaInstance_basic
--- PASS: TestAccRdsReadReplicaInstance_basic (1338.24s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       1338.320s