backend/s3: add custom_ca_bundle argument

[jar-b]

Adds a custom_ca_bundle argument to the S3 backend for parity with the AWS Provider.

% TF_ACC=1 go test -count=1 ./internal/backend/remote-state/s3/...
ok      github.com/hashicorp/terraform/internal/backend/remote-state/s3 107.640s

Closes #33689
Relates #33687

Target Release

1.6.x

Draft CHANGELOG entry

UPGRADE NOTES

• Adds a custom_ca_bundle argument and support for the corresponding AWS environment variable, AWS_CA_BUNDLE. (#33689)

[jar-b]

Closing in favor of #33765, which groups the remaining AWS provider parity enhancements into a single PR.

[github-actions]

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