Allow user to generate support bundle in Airgap environment

[2ez4szliu]

*Issue #, if available: 5783

*Description of changes:
Currently eks-anywhere cannot generate support bundle in Airgap environment because eks-a needs to read bundle manifest from remote URL.

This PR fixed the issue by allowing user to specify path of local bundle manifest file so that eks-a does not need to read it from remote URL.

*Testing (if applicable):
Run eksctl to generate support bundle with bundle-manifest flag specified in the command, bundle can be generated successfully.
eksctl-anywhere generate support-bundle -f eks-a-cluster.yaml --bundle-manifest eks-anywhere-downloads/bundle-release.yaml

Documentation added/planned (if applicable):

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

Attention: Patch coverage is 14.28571% with 6 lines in your changes missing coverage. Please review.

Project coverage is 73.52%. Comparing base (a083f41) to head (514322d).
Report is 59 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/eksctl-anywhere/cmd/supportbundle.go	14.28%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8683      +/-   ##
==========================================
- Coverage   73.53%   73.52%   -0.01%     
==========================================
  Files         578      578              
  Lines       36557    36561       +4     
==========================================
+ Hits        26882    26883       +1     
- Misses       7955     7958       +3     
  Partials     1720     1720              

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[jacobweinstock]

/approve

[eks-distro-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jacobweinstock

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[2ez4szliu]

/cherry-pick release-0.20

[eks-distro-pr-bot]

@2ez4szliu: new pull request created: #8786

In response to this:

/cherry-pick release-0.20

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