### What changes were proposed in this pull request?
This PR aims to add support for specifying a user defined initial state for arbitrary structured streaming stateful processing using [flat]MapGroupsWithState operator.
### Why are the changes needed?
Users can load previous state of their stateful processing as an initial state instead of redoing the entire processing once again.
### Does this PR introduce _any_ user-facing change?
Yes this PR introduces new API
```
def mapGroupsWithState[S: Encoder, U: Encoder](
timeoutConf: GroupStateTimeout,
initialState: KeyValueGroupedDataset[K, S])(
func: (K, Iterator[V], GroupState[S]) => U): Dataset[U]
def flatMapGroupsWithState[S: Encoder, U: Encoder](
outputMode: OutputMode,
timeoutConf: GroupStateTimeout,
initialState: KeyValueGroupedDataset[K, S])(
func: (K, Iterator[V], GroupState[S]) => Iterator[U])
```
### How was this patch tested?
Through unit tests in FlatMapGroupsWithStateSuite
Closes#33093 from rahulsmahadev/flatMapGroupsWithState.
Authored-by: Rahul Mahadev <rahul.mahadev@databricks.com>
Signed-off-by: Gengliang Wang <gengliang@apache.org>
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