Validate PostgreSQL ClickPipe inputs before Cloud API dispatch

[sdairs]

Summary

• validate PostgreSQL ClickPipe ports, required and non-empty table mappings, IAM role authentication, and replication-slot mode before Cloud API access
• extract a typed PostgreSQL request builder while preserving valid basic-auth cdc, snapshot, and cdc_only request shapes
• cover clap parsing, minimal/maximal typed requests, certificate and destination fields, and subprocess no-dispatch behavior

Verification

• cargo test -p clickhousectl cloud::clickpipes::tests (43 passed)
• cargo test -p clickhousectl --test cli_request_shape_test postgres_ (23 passed)
• cargo fmt --all --check
• cargo build -p clickhousectl
• cargo test -p clickhousectl (744 passed)
• cargo clippy -p clickhousectl --all-targets -- -D warnings

Stack

Position: 1 of 1, targeting main (bottom and top).

Closes #449

[iskakaushik]

LGTM

[sdairs]

Superseded by #539 in stack #563.