plugins: Use container_of() in doveadm functions instead of hard casts

src/plugins/doveadm-sieve/doveadm-sieve-cmd-activate.c

src/plugins/doveadm-sieve/doveadm-sieve-cmd-delete.c

src/plugins/doveadm-sieve/doveadm-sieve-cmd-get.c

src/plugins/doveadm-sieve/doveadm-sieve-cmd-put.c

src/plugins/doveadm-sieve/doveadm-sieve-cmd-rename.c

src/plugins/doveadm-sieve/doveadm-sieve-cmd.c