[IDEA]: ProtoBuf - make `license.id` an (external) enum

currently(CDX 1.6), we have the following situation:
- for JSON, the known SPDX licence IDs are in an own schema store:
- for XML, the known SPDX licence IDs are in an own schema store:
- for ProtoBuf, the known SPDX licence IDs are not an enum at all, they are just a free text(`string`):

## problem

Using arbitrary strings for `license.id` means, that there is just no baked-in safety.
To improve this, I propose to use a dedicated ProtoBuf package that can be maintained and released outside the regular CDX release cycle, just like the enum for JSON and XML.

## possible solution

- have a file `spdx.proto`
- use an own package `cyclonedx.spdx`
- declare the like
```proto
enum LicenseId {
LICENSEID_UNSPECIFIED = 0
// 0BSD
LICENSEID_0BSD = 1
// ...
// Apache-1.0
LICENSEID_Apache_1_0
// ...
}
- have this file checked against breaking changes
- see
- see
- have the license file updated with the other `spdx.*` schema files
- in the `bom-1.x.proto` file, use that enum (pseudocode)
```proto
import "cyclonedx.spdx";
message License {
oneof license {
// A known SPDX license identifier.
cyclonedx.spdx.LicenseId = 1;
// ...
}
// ...
}
```

## consideration & research

- ❗ this might be a breaking change - need to investigate
- ❕ need to investigate how/where to publish the schema file, so that has the intended effect