feat: add HUB_KUBE_HEADER system environment variable

[mingqishao]

What type of PR is this?
add a new feature: custom the header of hub's http client.

What this PR does / why we need it:
The hub cluster need additional http header for authentication or authorization. This PR to add a new system environment variable "HUB_KUBE_HEADER" to allow sending custom header to hub's API Server

Which issue(s) this PR fixes:

Fixes #

Requirements:

• run make reviewable for basic local test
• Read and followed fleet-networking's Code of conduct.
• uses conventional commit messages
• includes documentation
• adds unit tests

How has this code been tested

Special notes for your reviewer

[codecov]

Codecov Report

Merging #150 (dcd32a2) into main (5c58841) will increase coverage by 0.47%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##             main     #150      +/-   ##
==========================================
+ Coverage   78.50%   78.97%   +0.47%     
==========================================
  Files          20       21       +1     
  Lines        2735     2759      +24     
==========================================
+ Hits         2147     2179      +32     
+ Misses        468      460       -8     
  Partials      120      120              

Files Changed	Coverage Δ
pkg/common/hubconfig/hubconfig.go	78.46% <70.00%> (-1.54%)	⬇️
pkg/common/httpclient/round_tripper.go	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes

[michaelawyu]

LGTM