refactor: Optimize the process of cluster registration

[zhzhuang-zju]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:
Ref to #6214 (comment), the current cluster registration process has the following issues:

• Unreasonable function behavior
• Multiple execution branches
• Useless operations
• Relatively scattered processes, prone to issues such as unassigned parameters or null pointers.

In this refactoring, while preserving the original process, functions are aggregated and divided into three sub-modules: parameter initialization, parameter validation, and execution, which simplifies the behavior of callers.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[karmada-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
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Needs approval from an approver in each of these files:

• cmd/OWNERS
• pkg/karmadactl/OWNERS
• pkg/util/OWNERS

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[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 67.50000% with 65 lines in your changes missing coverage. Please review.

Project coverage is 45.61%. Comparing base (ea9bcec) to head (9af7086).
Report is 12 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/util/cluster.go	67.03%	45 Missing and 15 partials ⚠️
cmd/agent/app/agent.go	0.00%	3 Missing ⚠️
pkg/karmadactl/join/join.go	86.66%	0 Missing and 2 partials ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6499      +/-   ##
==========================================
- Coverage   48.98%   45.61%   -3.38%     
==========================================
  Files         687      686       -1     
  Lines       56097    56114      +17     
==========================================
- Hits        27481    25595    -1886     
- Misses      26836    28923    +2087     
+ Partials     1780     1596     -184     

Flag	Coverage Δ
unittests	45.61% <67.50%> (-3.38%)	⬇️

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