add huaweicloud vpcep endpoint resource and docs

[ShiChangkuo]

the testing result as follows:

$ make testacc TEST='./huaweicloud' TESTARGS='-run TestAccVPCEndpointBasic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run TestAccVPCEndpointBasic -timeout 360m -parallel 4
=== RUN   TestAccVPCEndpointBasic
=== PAUSE TestAccVPCEndpointBasic
=== CONT  TestAccVPCEndpointBasic
--- PASS: TestAccVPCEndpointBasic (200.07s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       200.105s
$
$ make testacc TEST='./huaweicloud' TESTARGS='-run TestAccVPCEndpointPublic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run TestAccVPCEndpointPublic -timeout 360m -parallel 4
=== RUN   TestAccVPCEndpointPublic
=== PAUSE TestAccVPCEndpointPublic
=== CONT  TestAccVPCEndpointPublic
--- PASS: TestAccVPCEndpointPublic (34.26s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       34.296s