feat: reuse connections and limit the number of connections for preheating

[gaius-qi]

Description

This pull request introduces several improvements to the manager/job/preheat.go file, primarily focusing on enhancing the HTTP client configuration and refactoring the code for better maintainability.

HTTP Client Configuration Improvements:

• Introduced a new defaultHTTPTransport variable to define the default HTTP transport settings, including maximum idle connections, maximum connections per host, and idle connection timeout.
• Modified the preheat struct to include an httpClient field, and updated the newPreheat function to initialize this client with the defaultHTTPTransport settings.
• Updated the getImageLayers method to use the httpClient from the preheat struct instead of creating a new HTTP client each time.

Code Refactoring:

• Refactored the newImageAuthClient function to use the defaultHTTPTransport for its HTTP client, ensuring consistent transport settings across different parts of the codebase.

Related Issue

Fixed #3635.

Motivation and Context

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation Update (if none of the other choices apply)

Checklist

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[chlins]

lgtm

[codecov]

Codecov Report

Attention: Patch coverage is 62.96296% with 10 lines in your changes missing coverage. Please review.

Project coverage is 35.33%. Comparing base (a97584a) to head (65e9cd3).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
manager/job/preheat.go	62.96%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3683      +/-   ##
==========================================
- Coverage   35.39%   35.33%   -0.06%     
==========================================
  Files         335      335              
  Lines       37631    37629       -2     
==========================================
- Hits        13318    13296      -22     
- Misses      23488    23503      +15     
- Partials      825      830       +5     

Flag	Coverage Δ
unittests	35.33% <62.96%> (-0.06%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
manager/job/preheat.go	46.06% <62.96%> (-3.75%)	⬇️

... and 3 files with indirect coverage changes