feat(cloud): expose service settings management

[sdairs]

Expose all five beta per-service ClickHouse settings endpoints as cloud service settings list|get|schema|set|unset. Read commands support OAuth, while set and unset follow the existing API-key-only write policy.

settings set accepts repeatable --setting NAME=JSON_VALUE arguments or a JSON settings map from --settings-file (- reads stdin). It preserves JSON value types and serializes that map into the API's required JSON-encoded settings string; malformed JSON and accidental full request wrappers fail before HTTP. Setting names remain dynamic and are validated by the API, with settings schema providing discovery. settings unset idempotently restores one setting's platform default.

Tests cover clap parsing and write classification, minimal and varied typed request construction, plain file input and wrapper rejection, all five real-binary routes, OAuth and API-key authentication, exact PATCH encoding, sparse JSON and human output, API errors, and invalid stdin before HTTP.

Validation:

• cargo fmt --all --check
• cargo clippy -p clickhousectl -- -D warnings
• cargo check -p clickhousectl --no-default-features
• cargo clippy -p clickhousectl --all-targets --no-default-features -- -D warnings
• cargo test -p clickhousectl -- --test-threads=1

Closes #574

[iskakaushik]

LGTM