[vtctldclient] Add GetShardReplication

[ajm188]

Description

Its omission was discovered in #15297.

I'd like to backport this to 19.0 while the release window is still open.

Demo

vtctldclient --server :15999 GetShardReplication commerce/0
{
  "shard_replication_by_cell": {
    "zone1": {
      "nodes": [
        {
          "tablet_alias": {
            "cell": "zone1",
            "uid": 100
          }
        },
        {
          "tablet_alias": {
            "cell": "zone1",
            "uid": 101
          }
        },
        {
          "tablet_alias": {
            "cell": "zone1",
            "uid": 102
          }
        }
      ]
    }
  }
}

Related Issue(s)

Closes #15386

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 24 lines in your changes are missing coverage. Please review.

Project coverage is 65.46%. Comparing base (15f5886) to head (477bfd1).

Files	Patch %	Lines
go/cmd/vtctldclient/command/shards.go	5.26%	18 Missing ⚠️
go/vt/vtctl/grpcvtctldclient/client_gen.go	0.00%	4 Missing ⚠️
go/vt/vtctl/localvtctldclient/client_gen.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15389      +/-   ##
==========================================
+ Coverage   65.44%   65.46%   +0.01%     
==========================================
  Files        1562     1562              
  Lines      193863   193911      +48     
==========================================
+ Hits       126880   126938      +58     
+ Misses      66983    66973      -10     

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