Custom Licenses

Currently SPDX license identifiers are encouraged, with https://github.com/cps-org/cps/pull/22 required.
SPDX defines `LicenseRef-.*` to use custom licenses (See [here](https://spdx.github.io/spdx-spec/v2.2.2/SPDX-license-expressions/#d3-simple-license-expressions) and [here](https://spdx.github.io/spdx-spec/v2.2.2/other-licensing-information-detected/)).
I think cps should also define something so that custom license ids could be defined and used in the license expression so that custom licenses could be represented.
We could use the fields from https://spdx.github.io/spdx-spec/v2.2.2/other-licensing-information-detected/:
Maybe something like this:
```json
{
"Name": "sample",
"Description": "Sample CPS",
"License": "LicenseRef-MyLicense",
"license-definitions": {
"LicenseRef-MyLicense": {
"name": "MyLicense maybe with special characters",
"see-also": "http://example.com/licenses/our-license",
"comment": "This is a comment",
"extracted-text": "Do whatever you want with this code"
}
}
}
```