Adds Custom to RSI Licenses

[Redrover1760]

Allows forks to use Custom licensing for RSI

without having to edit engine files

I understand wanting to block this for standard wizden servers, however, testfailing on all servers is bad and and if its still desired it should be moved to content or something

[ike709]

The RSI specification doesn't actually explicitly support Custom like the RGA spec does.

Current RSI pertinent text:

license | Required. A valid SPDX License Identifier applying to this work.

Current RGA pertinent text:

license | A valid SPDX License Identifier applying to all files within an entry. If a license does not have a valid SPDX identifier, Custom may be used but a link to the license should be provided in the copyright field.

Should probably copy the RGA version of the text over to the RSI specification document?