add huaweicloud vpcep service resource and docs

[ShiChangkuo]

the testting result as follows:

$ make testacc TEST='./huaweicloud' TESTARGS='-run TestAccVPCEPServiceBasic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run TestAccVPCEPServiceBasic -timeout 360m -parallel 4
=== RUN   TestAccVPCEPServiceBasic
=== PAUSE TestAccVPCEPServiceBasic
=== CONT  TestAccVPCEPServiceBasic
--- PASS: TestAccVPCEPServiceBasic (200.97s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       201.028s