feat: Set ReadonlyRootFilesystem to true by default in manifest

[KollaAdithya]

This PR resolves #4010

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the Apache 2.0 License.

[efekarakus]

Looks great ❤️, thanks Adi!!!

[codecov-commenter]

Codecov Report

Merging #4062 (451871e) into mainline (9a6d5d9) will increase coverage by 0.11%. The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           mainline    #4062      +/-   ##
============================================
+ Coverage     68.99%   69.10%   +0.11%     
============================================
  Files           248      248              
  Lines         35294    35461     +167     
  Branches        264      264              
============================================
+ Hits          24350    24505     +155     
- Misses         9752     9763      +11     
- Partials       1192     1193       +1     

Impacted Files	Coverage Δ
internal/pkg/template/workload.go	42.58% <ø> (+3.84%)	:arrow_up:
...al/pkg/deploy/cloudformation/stack/transformers.go	76.95% <100.00%> (+0.03%)	:arrow_up:
internal/pkg/manifest/storage.go	93.58% <100.00%> (ø)
internal/pkg/manifest/validate.go	84.43% <100.00%> (+0.08%)	:arrow_up:
internal/pkg/manifest/env.go	78.76% <0.00%> (-1.87%)	:arrow_down:
...ternal/pkg/deploy/cloudformation/stack/workload.go	77.21% <0.00%> (-0.52%)	:arrow_down:
internal/pkg/cli/svc_package.go	26.39% <0.00%> (-0.46%)	:arrow_down:
internal/pkg/deploy/cloudformation/stack/env.go	71.96% <0.00%> (-0.21%)	:arrow_down:
...rnal/pkg/deploy/cloudformation/stack/lb_web_svc.go	77.51% <0.00%> (+0.08%)	:arrow_up:
internal/pkg/cli/deploy/svc.go	48.27% <0.00%> (+0.10%)	:arrow_up:
... and 10 more

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