Load balancer support import

[Lance52259]

What this PR does / why we need it:
After load balancer supports the import function, users can use existing load balancer resources to build ELB-related services.

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

NONE

Special notes for your reviewer:

Release note:

1. load balancer support the import function. 
2. add import test.

PR Checklist

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

Acceptance Steps Performed

make testacc TEST='./huaweicloud' TESTARGS='-run=TestAccLBV2LoadBalancer'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run=TestAccLBV2LoadBalancer -timeout 360m -parallel 4
=== RUN   TestAccLBV2LoadBalancer_basic
=== PAUSE TestAccLBV2LoadBalancer_basic
=== RUN   TestAccLBV2LoadBalancer_secGroup
=== PAUSE TestAccLBV2LoadBalancer_secGroup
=== RUN   TestAccLBV2LoadBalancer_withEpsId
=== PAUSE TestAccLBV2LoadBalancer_withEpsId
=== CONT  TestAccLBV2LoadBalancer_basic
=== CONT  TestAccLBV2LoadBalancer_withEpsId
=== CONT  TestAccLBV2LoadBalancer_secGroup
--- PASS: TestAccLBV2LoadBalancer_withEpsId (51.49s)
--- PASS: TestAccLBV2LoadBalancer_basic (82.06s)
--- PASS: TestAccLBV2LoadBalancer_secGroup (101.30s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       101.353s