Add pkg-config cataloger

**What would you like to be added**:

On traditional UNIX platforms, installed SDKs frequently install themselves in the pkg-config database. It would be useful to catalog these SDKs as part of an SBOM.

**Why is this needed**:

By checking the pkg-config versions of a SDK, we can detect vulnerable dependencies which register themselves with the pkg-config database.

**Additional context**:

I happen to have a pkg-config database parser at https://github.com/chainguard-dev/go-pkgconfig :)

Start by reviewing Syft's existing catalogers and the referenced go-pkgconfig parser for how a pkg-config database is read. Define the cataloger's input and output around SDK versions registered in pkg-config, then verify that discovered dependencies can support vulnerability detection and SBOM generation.

Written by the indexing model from the issue text.