backend/s3: Move assume role parameters to block

[gdavison]

The assume role parameters are currently at the root level.

1. Add block assume_role with

• duration of type duration
• external_id
• policy
• policy_arns
• role_arn (required)
• session_name
• tags
• transitive_tag_keys

2. Deprecate

• role_arn
• session_name
• external_id
• assume_role_duration_seconds
• assume_role_policy
• assume_role_policy_arns
• assume_role_tags
• assume_role_transitive_tag_keys

[blowfishpro]

It seems like this will break the ability to set these variables via CLI arguments (e.g. -backend-config=role_arn=...) or via environment variables (e.g. TF_CLI_ARGS_init='-backend-config=role_arn=...'). Before this is removed entirely is there any chance #21830 could be addressed so that block variables can be set via -backend-config, or that specific environment variables could be added for these? (or #13022 addressed but I assume that's a much deeper change within terraform)

[github-actions]

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