feat(dli): add new datasource to get list of elastic resource pools

[wuzhuanhong]

What this PR does / why we need it:

Add new datasource to get list of elastic resource pools.

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:

 1. support new datasource to get list of elastic resource pools.
 2. support related document and acceptance test.

PR Checklist

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

Acceptance Steps Performed

make testacc TEST='./huaweicloud/services/acceptance/dli' TESTARGS='-run TestAccDataSourceDliElasticResourcePools_basic'
==> Checking that code complies with gofmt requirements...

TF_ACC=1 go test ./huaweicloud/services/acceptance/dli -v -run TestAccDataSourceDliElasticResourcePools_basic-timeout 360m -parallel 4

=== RUN   TestAccDataSourceDliElasticResourcePools_basic
=== PAUSE TestAccDataSourceDliElasticResourcePools_basic
=== CONT  TestAccDataSourceDliElasticResourcePools_basic
--- PASS: TestAccDataSourceDliElasticResourcePools_basic(4585.20s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/dli       4585.675s