feat: Separates Offsets from different context (Spark vs PSL)

[jiangmichaellll]

Spark offset means msgs with offsets less than or equal to have been processed, and next to be delivered is spark_offset+1.
PSL offset means msgs with offsets less than have been processed, and next to be delivered is psl_offset.

Spark side:
SparkSourceOffset, SparkPartitionOffset
PSL side:
PslSourceOffset, PslPartitionOffset

Also added utils method to convert between PSL and Spark.

[codecov]

Codecov Report

Merging #404 (7356b05) into master (8f26f54) will increase coverage by 0.09%.
The diff coverage is 86.58%.

@@             Coverage Diff              @@
##             master     #404      +/-   ##
============================================
+ Coverage     71.85%   71.95%   +0.09%     
- Complexity      828      833       +5     
============================================
  Files           156      158       +2     
  Lines          4356     4457     +101     
  Branches        221      223       +2     
============================================
+ Hits           3130     3207      +77     
- Misses         1106     1125      +19     
- Partials        120      125       +5     

Impacted Files	Coverage Δ	Complexity Δ
...google/cloud/pubsublite/spark/PslSourceOffset.java	76.92% <72.72%> (+0.25%)	5.00 <5.00> (-6.00)	⬆️
...ogle/cloud/pubsublite/spark/SparkSourceOffset.java	85.10% <85.10%> (ø)	17.00 <17.00> (?)
...m/google/cloud/pubsublite/spark/PslSparkUtils.java	59.37% <95.00%> (+59.37%)	6.00 <5.00> (+6.00)
...gle/cloud/pubsublite/spark/PslPartitionOffset.java	100.00% <100.00%> (ø)	2.00 <1.00> (ø)
...e/cloud/pubsublite/spark/SparkPartitionOffset.java	100.00% <100.00%> (ø)	2.00 <2.00> (?)
...e/v1/stub/GrpcPublisherServiceCallableFactory.java	25.00% <0.00%> (-25.00%)	2.00% <0.00%> (-2.00%)
.../v1/stub/GrpcSubscriberServiceCallableFactory.java	25.00% <0.00%> (-25.00%)	2.00% <0.00%> (-2.00%)
...GrpcPartitionAssignmentServiceCallableFactory.java	25.00% <0.00%> (-25.00%)	2.00% <0.00%> (-2.00%)
...google/cloud/pubsublite/v1/AdminServiceClient.java	74.13% <0.00%> (-17.25%)	45.00% <0.00%> (-10.00%)
.../v1/stub/GrpcTopicStatsServiceCallableFactory.java	25.00% <0.00%> (-12.50%)	2.00% <0.00%> (-1.00%)
... and 47 more

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[dpcollins-google]

LGTM other than don't write your own equals or hashcode methods. Ping me when you've fixed that issue