Add keyless query result entities for headers and bodies

Added `HeaderQueryResult` and `BodyQueryResult` classes to represent
query results for headers and bodies, respectively. These classes
map their properties to database columns (`REQUEST_HEADER` and
`REQUEST_BODY`) using the `[Column]` attribute. Both properties
are nullable and immutable.

Updated `RecDbContext` to configure these entities as keyless
using the `HasNoKey()` method in the `OnModelCreating` method,
indicating they are used for read-only queries.

src/ReC.Infrastructure/RecDbContext.cs

@@ -16,5 +16,9 @@ public class RecDbContext(DbContextOptions<RecDbContext> options) : DbContext(op
         base.OnModelCreating(modelBuilder);
 
         modelBuilder.Entity<RecAction>().HasNoKey();
+
+        modelBuilder.Entity<HeaderQueryResult>().HasNoKey();
+
+        modelBuilder.Entity<BodyQueryResult>().HasNoKey();
     }
 }