backend/s3: support multiple shared config/credentials files

[jar-b]

Adds the shared_config_files and shared_credentials_files parameters and adds support for default AWS environment variables for credentials and configuration files, AWS_CONFIG_FILE and AWS_SHARED_CREDENTIALS_FILE. This deprecates the shared_credentials_file parameter.

% TF_ACC=1 go test -count=1 ./internal/backend/remote-state/s3/...
ok      github.com/hashicorp/terraform/internal/backend/remote-state/s3 102.007s

Relates #30493

Target Release

1.6.0

Draft CHANGELOG entry

UPGRADE NOTES

• Supports the default AWS environment variables for credentials and configuration files: AWS_CONFIG_FILE and AWS_SHARED_CREDENTIALS_FILE. [backend/s3: Add multiple credentials and config files parameters #30493]
• Adds shared_config_files and shared_credentials_files parameters. This deprecates the shared_credentials_file parameter. [backend/s3: Add multiple credentials and config files parameters #30493]

[gdavison]

Looks good! 🚀

[github-actions]

Reminder for the merging maintainer: if this is a user-visible change, please update the changelog on the appropriate release branch.

[github-actions]

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