feat: add GetNamespace utils method for context [multi-tenancy PR 1]

[binbin-li]

Description

What this PR does / why we need it:

This is the first PR for multi-tenancy support.

1. Add a GetNamespace util method. It fetches the namespace embeded in the context. For each request, we'll use SetContextWithNamespace to embed the namespace into the context, and consumers like executors, verifiers, policyEnforcers could get the namespace from the context instead of passing it through function param explicitely.
2. Embed the namespace into the context for mutation requests as well.

Which issue(s) this PR fixes (optional, using fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when the PR gets merged):

Fixes #

Type of change

Please delete options that are not relevant.

• 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 not work as expected)
• Helm Chart Change (any edit/addition/update that is necessary for changes merged to the main branch)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Please also list any relevant details for your test configuration

• Test A
• Test B

Checklist:

• Does the affected code have corresponding tests?
• Are the changes documented, not just with inline documentation, but also with conceptual documentation such as an overview of a new feature, or task-based documentation like a tutorial? Consider if this change should be announced on your project blog.
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have appropriate license header?

Post Merge Requirements

• MAINTAINERS: manually trigger the "Publish Package" workflow after merging any PR that indicates Helm Chart Change

[codecov]

Codecov Report

Attention: Patch coverage is 72.72727% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 65.14%. Comparing base (45c0f81) to head (bb1470c).
Report is 38 commits behind head on main.

Files	Patch %	Lines
httpserver/handlers.go	50.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1356      +/-   ##
==========================================
+ Coverage   61.60%   65.14%   +3.54%     
==========================================
  Files          98      104       +6     
  Lines        6248     5440     -808     
==========================================
- Hits         3849     3544     -305     
+ Misses       2066     1538     -528     
- Partials      333      358      +25     

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[akashsinghal]

lgtm. Thanks for walking through PR in the PR review meeting.

[susanshi]

thanks for updating the PR description!