security hooks on object creation

The attached patch provides plugin modules a hook just after object
creation time. In typical use cases, it enables to assign default
security labels on object creation by the external security providers.

As Robert suggested before, it provides a generic purpose main hook.
It takes an enum of ObjectAccessType which informs plugins what kind
of accesses are required, and identifier of the object to be referenced.
But, in this version, no additional information, such as new name in
ALTER xxx RENAME TO, are not supported.

The ObjectAccessType is defined as follows:

typedef enum ObjectAccessType {
OAT_POST_CREATE, /* Post creation fixups; such as security labeling */
} ObjectAccessType;

We will support more complete kind of access types in the future version,
however, we focus on default labeling rather than DDL permissions right
now, so only OAT_POST_CREATE is defined here.
Perhaps, we will add OAT_ALTER, OAT_DROP, OAT_COMMENT and so on.

In this patch, I put hooks on the place just after creation of database
objects that we can assign security labels. (schema, relation, attribute,
procedure, language, type, large object)

However, I didn't touch or move CommandCounterIncrement() yet, although
we had a long discussion MVCC visibility of new object.
Because I'm not clear whether it is really preferable to inject CCIs
onto random points such as TypeCreate() or ProcedureCreate() under
development of the version killed by myself.
(In other words, it was simply ugly...)

At least, we can see the new entries with SnapshotSelf, although we will
pay performance penalty. If so, it is an idea not to touch anything
related to CCIs.
The purpose of post creation hooks are assignment of default security
labels, not DDL permissions. So, it is not a bad idea not to touch
routines related to CCIs in the earlier version of external security
provider.

In this patch, we put InvokeObjectAccessHook0 on the following functions.

- heap_create_with_catalog() for relations/attributes
- ATExecAddColumn() for attributes
- NamespaceCreate() for schemas
- ProcedureCreate() for aggregates/functions
- TypeCreate() and TypeShellMake() for types
- create_proc_lang() for procedural languages
- inv_create() for large objects

Thanks,
--
KaiGai Kohei <kaigai(at)ak(dot)jp(dot)nec(dot)com>