BaseTools: Add support for SUBTYPE_GUID section generation

BaseTools: Add support for SUBTYPE_GUID section generation

EFI_SECTION_FREEFORM_SUBTYPE_GUID is a leaf section type that contains
a single EFI_GUID in the header to describe the raw data.
Currently is is not possible to generate such section.
This patch adds initial support for the generation of such sections.
The added syntax for this type of section corresponds to EDKII
"[FV] section" documentation from the FDF Specification:
```
SECTION SUBTYPE_GUID <GUID> = <File>
```

Signed-off-by: Konstantin Aladyshev <aladyshev22@gmail.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>