RFE: Permit adding arbitrary tag types in headers

[n3npq]

Parallel to adding arbitrary tagno's by computing a hash from a canonicalized string, rpm should permit adding arbitrary data types in header tags.

The default "unknown" accessor should return an octet blob type with a count computed from offsets in the data area (hence leaving the count field to be used for other purposes, such as size of an array item, to return item count for an array of items like sizeof(array)/sizeof(array[0]) analogue to C code).

The default alignment for arbitrary tag types in the data area should be 8

The biggest impediment to implementing arbitrary tag types will be getting past the sanity check on the tag type range (which has just been fixed).