Fix nil pointer errors and updated the keys new URL path

[Yashsharma1911]

Notes for Reviewers

This PR fixes #9721

We already exposed keys in Meshery, however since there are new updates in URL I updated the path in this PR, also there were a couple of nil pointer errors coming which this PR also fixes.

I created a new handler specific for keys to not increase extra LOC in only users handlers

Signed commits

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[codecov]

Codecov Report

Attention: 17 lines in your changes are missing coverage. Please review.

Comparison is base (71c6b21) 10.15% compared to head (9a8b520) 10.12%.
Report is 33 commits behind head on master.

Files	Patch %	Lines
server/handlers/keys_handler.go	0.00%	16 Missing ⚠️
server/router/server.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9731      +/-   ##
==========================================
- Coverage   10.15%   10.12%   -0.04%     
==========================================
  Files         139      140       +1     
  Lines       22115    22187      +72     
==========================================
  Hits         2246     2246              
- Misses      19514    19586      +72     
  Partials      355      355              

Flag	Coverage Δ
unittests	10.12% <0.00%> (-0.04%)	⬇️

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[Yashsharma1911]

// @theBeginner86