Fix: alter PAX table will got cast problem

PAX needs to define the StdRdOptions instead of vl_len.
Because many places in CBDB assume that option in relation can be cast into StdRdOptions.

Author: jiaqizho

contrib/pax_storage/expected/ddl.out

@@ -63,3 +63,9 @@ vacuum pax_test.t1;
 vacuum full pax_test.t1;
 drop table pax_test.t1;
 drop table pax_test.t2;
+-- alter column with options 
+create table pax_test.t3 (v1 numeric(100,1)) with(compresstype=zstd, compresslevel=1);
+NOTICE:  Table doesn't have 'DISTRIBUTED BY' clause -- Using column named 'v1' as the Cloudberry Database data distribution key for this table.
+HINT:  The 'DISTRIBUTED BY' clause determines the distribution of data. Make sure column(s) chosen are the optimal data distribution key to minimize skew.
+alter table pax_test.t3 alter column v1 type numeric;
+drop table pax_test.t3;

contrib/pax_storage/sql/ddl.sql

@@ -41,3 +41,7 @@ vacuum full pax_test.t1;
 drop table pax_test.t1;
 drop table pax_test.t2;
 
+-- alter column with options 
+create table pax_test.t3 (v1 numeric(100,1)) with(compresstype=zstd, compresslevel=1);
+alter table pax_test.t3 alter column v1 type numeric;
+drop table pax_test.t3;

contrib/pax_storage/src/cpp/access/paxc_rel_options.h

@@ -31,7 +31,10 @@ namespace paxc {
 
 // plain structure used by reloptions, can be accessed from C++ code.
 struct PaxOptions {
-  int32 vl_len; /* varlena header (do not touch directly!) */
+  // Pax needs to define the StdRdOptions instead of just vl_len. 
+  // This is because many places in the CBDB assume that option in 
+  // relation can be cast into StdRdOptions.
+  StdRdOptions rd_options; 
   char storage_format[16];
   char compress_type[16];
   int compress_level;