frp: migrate generated configuration from INI to TOML

frp: migrate generated configuration from INI to TOML

Migrate frpc and frps configuration generation to the TOML format used by
current frp releases.

- generate TOML configuration from UCI options
- add UCI defaults migration for existing installations
- install additional fragments under /etc/frp/frpc.d and /etc/frp/frps.d
- preserve manual raw TOML fragment support without exposing it in LuCI
- keep client and server TCP mux settings aligned
- update the package metadata for the current frp release artifacts

Signed-off-by: Roc Lai <redacted>