### What changes were proposed in this pull request?
This patch is a follow-up of SPARK-26848 (#23747). In SPARK-26848, we decided to open possibility to let end users set individual timestamp per partition. But in many cases, specifying timestamp represents the intention that we would want to go back to specific timestamp and reprocess records, which should be applied to all topics and partitions.
This patch proposes to provide a way to set a global timestamp across topic-partitions which the source is subscribing to, so that end users can set all offsets by specific timestamp easily. To provide the way to config the timestamp easier, the new options only receive "a" timestamp for start/end timestamp.
New options introduced in this PR:
* startingTimestamp
* endingTimestamp
All two options receive timestamp as string.
There're priorities for options regarding starting/ending offset as we will have three options for start offsets and another three options for end offsets. Priorities are following:
* starting offsets: startingTimestamp -> startingOffsetsByTimestamp -> startingOffsets
* ending offsets: startingTimestamp -> startingOffsetsByTimestamp -> startingOffsets
### Why are the changes needed?
Existing option to specify timestamp as offset is quite verbose if there're a lot of partitions across topics. Suppose there're 100s of partitions in a topic, the json should contain 100s of times of the same timestamp.
Also, the number of partitions can also change, which requires either:
* fixing the code if the json is statically created
* introducing the dependencies on Kafka client and deal with Kafka API on crafting json programmatically
Both approaches are even not "acceptable" if we're dealing with ad-hoc query; anyone doesn't want to write the code more complicated than the query itself. Flink [provides the option](https://ci.apache.org/projects/flink/flink-docs-release-1.13/docs/connectors/datastream/kafka/#kafka-consumers-start-position-configuration) to specify a timestamp for all topic-partitions like this PR, and even doesn't provide the option to specify the timestamp per topic-partition.
With this PR, end users are only required to provide a single timestamp value. No more complicated JSON format end users need to know about the structure.
### Does this PR introduce _any_ user-facing change?
Yes, this PR introduces two new options, described in above section.
Doc changes are following:
### How was this patch tested?
New UTs covering new functionalities. Also manually tested via simple batch & streaming queries.
Closes#32609 from HeartSaVioR/SPARK-29223-v2.
Authored-by: Jungtaek Lim <kabhwan.opensource@gmail.com>
Signed-off-by: Jungtaek Lim <kabhwan.opensource@gmail.com>
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