add a flag to determine whether `ReadContext` is called via `schema.ImportStatePassthroughContext`

### SDK version

```
github.com/hashicorp/terraform-plugin-sdk/v2 v2.18.0dk/...
```

### Use-cases

currently, there is no way to determine whether we are encountering an on-going import operation in `schema.Resource.ReadContext` if it is called via `schema.ImportStatePassthroughContext`

### Attempted Solutions

there is, currently, no clean solution to solve this

there is, however, a way to achieve it by implementing custom `schema.Resource.Importer.StateContext`, however we would like to be using `schema.ImportStatePassthroughContext`

### Proposal

add a method to `schema.ResourceData`, similar to `IsNewResource`, which would return `true` if there is an on-going import operation

Start by tracing schema.ImportStatePassthroughContext into schema.Resource.ReadContext and inspect ResourceData methods such as IsNewResource. The change is complete when ResourceData exposes whether an import is ongoing and the behavior is covered by relevant SDK tests.

Written by the indexing model from the issue text.