helper/schema: Remove timeouts during ImportResourceState

[bflad]

Closes #1145
Reference: hashicorp/terraform#32463

Terraform 1.3.8 and later now correctly handles null values for single nested blocks. This means Terraform will now report differences between a null block and known block with null values. This SDK only supported single nested blocks via its timeouts functionality.

This change is a very targeted removal of any potential timeouts block values in a resource state from the ImportResourceState RPC. Since configuration is not available during that RPC, it is never valid to return any data beyond null for that block. This will prevent unexpected differences on the first plan after import, where Terraform will report the block removal for configurations which do not contain the block.

New unit test failure prior to code updates:

--- FAIL: TestImportResourceState_Timeouts_Removed (0.00s)
    /Users/bflad/src/github.com/hashicorp/terraform-plugin-sdk/helper/schema/grpc_provider_test.go:1159: unexpected difference:   cty.Value(
        - 	{
        - 		ty: cty.Type{typeImpl: cty.typeObject{AttrTypes: map[string]cty.Type{...}}},
        - 		v:  map[string]any{"id": string("test"), "string_attribute": nil, "timeouts": nil},
        - 	},
        + 	{
        + 		ty: cty.Type{typeImpl: cty.typeObject{AttrTypes: map[string]cty.Type{...}}},
        + 		v: map[string]any{
        + 			"id":               string("test"),
        + 			"string_attribute": nil,
        + 			"timeouts":         map[string]any{"create": nil, "read": nil},
        + 		},
        + 	},
          )

[bflad]

This cannot be verified via provider acceptance testing on Terraform 1.3.8 as TestStep.ImportStateVerify is explicitly coded to ignore timeouts differences. 😖 Rather than dealing with removing that special testing logic, I verified this downstream by rebuilding terraform-provider-aws, using developer overrides, and running an aws_security_group import and plan.

$ terraform import aws_security_group.test sg-0329f3c7cf561ad15                                                       
aws_security_group.test: Importing from ID "sg-0329f3c7cf561ad15"...
aws_security_group.test: Import prepared!
  Prepared aws_security_group for import
aws_security_group.test: Refreshing state... [id=sg-0329f3c7cf561ad15]

Import successful!

The resources that were imported are shown above. These resources are now in
your Terraform state and will henceforth be managed by Terraform.

$ terraform plan
╷
│ Warning: Provider development overrides are in effect
│ 
│ The following provider development overrides are set in the CLI configuration:
│  - hashicorp/aws in /Users/bflad/go/bin
│ 
│ The behavior may therefore not match any released version of the provider and applying changes may cause the state to become incompatible with published releases.
╵
aws_security_group.test: Refreshing state... [id=sg-0329f3c7cf561ad15]

No changes. Your infrastructure matches the configuration.

Terraform has compared your real infrastructure against your configuration and found no differences, so no changes are needed.

[jbardin]

I bet the timeouts are being added in HCL2ValueFromFlatmap which doesn't make use of the schema, and NormalizeObjectFromLegacySDK doesn't try to strip them out because it's a technically valid value. One of those functions should have been where this was done originally, but it probably doesn't matter so much at this point.

[github-actions]

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