Define file subtype value behavior/expectations

[skliper]

Is your feature request related to a problem? Please describe.
Not clear if the CFE_FS_InitHeader SubType needs to be one of the FS enums or if it can be user defined by apps. Note there is no longer a shell file created by ES:

cFE/modules/core_api/fsw/inc/cfe_fs_extern_typedefs.h

Lines 101 to 108 in e80aae9

	/**
	* @brief Executive Services Shell Response File
	*
	* Executive Services Shell Response Data File which is generated in response to a
	* shell command.
	*
	*/
	CFE_FS_SubType_ES_SHELL = 5,

Describe the solution you'd like
Need to determine if FS should define all file subtypes, or treat it as an extendable field (or whatever). That will affect if the SHELL subtype gets removed or renamed (since there is still an app that would create it). Note right now apps don't even use CFE_FS_InitHeader, but they do currently set unique values.

Describe alternatives you've considered
None

Additional context
Code review

Requester Info
Jacob Hageman - NASA/GSFC

[skliper]

Ping @ejtimmon

[astrogeco]

CCB:2021-04-21 DISCUSSED

[skliper]

The SubTypes defined within the cFE FS header are cFS framework only, and apps are allowed to extend. Could be future work issue to add app context to the header to avoid conflict.