Add ability to append labels to SBOM

**What would you like to be added**:
When generating (and especially uploading) an SBOM I want to collect meta-data about where the command was run, who ran it, or other relevant information that could be used later.

**Why is this needed**:
To provide a building block so that wrapping tools can help differentiate where and how an SBOM was generated.

**Additional context**:
I'd imagine this could be amended to the /.syft.yaml config e.g.

```
sbom-labels:
name: value
```

Start with the /.syft.yaml configuration example and trace how generation and uploading consume configuration. Define how sbom-labels name/value entries should be carried into generated and uploaded SBOMs, then verify both paths support metadata for wrapper tools.

Written by the indexing model from the issue text.