[ISSUE #9246] Support init offset mode in PopConsumerService

[lizhimins]

Which Issue(s) This PR Fixes

Fixes #9246

Brief Description

How Did You Test This Change?

[lollipopjin]

LGTM

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 35.29412% with 11 lines in your changes missing coverage. Please review.

Project coverage is 48.02%. Comparing base (622c807) to head (9baa23c).
Report is 47 commits behind head on develop.

Files with missing lines	Patch %	Lines
...apache/rocketmq/broker/pop/PopConsumerService.java	25.00%	7 Missing and 2 partials ⚠️
...rocketmq/broker/processor/PopMessageProcessor.java	0.00%	2 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop    #9247      +/-   ##
=============================================
- Coverage      48.06%   48.02%   -0.05%     
+ Complexity     12102    12092      -10     
=============================================
  Files           1322     1322              
  Lines          93105    93115      +10     
  Branches       11936    11938       +2     
=============================================
- Hits           44753    44717      -36     
- Misses         42811    42862      +51     
+ Partials        5541     5536       -5     

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