feat(repository): Add Custom Root CA option for S3

[babs]

Closes #1443

example:

kopia repository connect s3 \
  --access-key the-access-key \
  --secret-access-key the-secret-key \
  --bucket my-backup-bucket \
  --endpoint localhost:9000 \
  --root-ca-pem-base64=$(cat public.crt | base64 -w0)

ROOT_CA_PEM_BASE64=$(cat public.crt | base64 -w0)
kopia repository connect s3 \
  --access-key the-access-key \
  --secret-access-key the-secret-key \
  --bucket my-backup-bucket \
  --endpoint localhost:9000

kopia repository connect s3 \
  --access-key the-access-key \
  --secret-access-key the-secret-key \
  --bucket my-backup-bucket \
  --endpoint localhost:9000 \
  --root-ca-pem-path=public.crt

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.04 🎉

Comparison is base (7e9b3dc) 75.27% compared to head (ad88423) 75.31%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2845      +/-   ##
==========================================
+ Coverage   75.27%   75.31%   +0.04%     
==========================================
  Files         457      457              
  Lines       35901    35923      +22     
==========================================
+ Hits        27024    27057      +33     
+ Misses       6984     6976       -8     
+ Partials     1893     1890       -3     

Impacted Files	Coverage Δ
cli/storage_s3.go	74.50% <100.00%> (+19.33%)	⬆️

... and 7 files with indirect coverage changes

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[ntolia]

@jkowalski Do you have suggestions for testing this better? CLI tests are hard.

[babs]

I've something almost ready :)
Give me some time to finish it and I'll update the PR this evening :)

[babs]

I think this one should start to look nice :)