[ISSUE #7781] Get namespace from gRPC header

[drpmma]

Which Issue(s) This PR Fixes

Fixes #7781

Brief Description

Get namespace from gRPC x-mq-namespace header.

How Did You Test This Change?

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (92e058c) 43.25% compared to head (14b57fd) 43.21%.

Files	Patch %	Lines
...org/apache/rocketmq/proxy/common/ProxyContext.java	66.66%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop    #7782      +/-   ##
=============================================
- Coverage      43.25%   43.21%   -0.04%     
+ Complexity      9871     9870       -1     
=============================================
  Files           1173     1173              
  Lines          85030    85034       +4     
  Branches       11013    11013              
=============================================
- Hits           36782    36751      -31     
- Misses         43681    43717      +36     
+ Partials        4567     4566       -1     

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