Export specific format versions (SPDX)

[kzantow]

What would you like to be added:
Ability to export formats at a specific version, for example SPDX version 2.2.

Why is this needed:
Improve interoperability with other tools.

Additional context:
The leading thought is that this will be an extension of the format string, with an @<version-matcher> at the end, so you could run:

syft <something> -o [email protected]

which would output an SPDX 2.2 document version, or:

syft <something> -o spdx-tag-value@2

which would output the latest SPDX v2 document type (in this case would be 2.3).

Also: we will probably only be able to implement this for SPDX at the moment, since the spdx/tools-golang library now has support for conversions between different versions but other libraries do not (including Syft JSON itself!).