pkg-config rules

When converting a target from `rules_foreign_cc` to `rules_cc` it was revealed that this dropped integrations with other `rules_foreign_cc` targets (https://github.com/bazelbuild/bazel-central-registry/pull/8393#issuecomment-4250284232). The issue is `ffmpeg` uses [pkg-config](https://linux.die.net/man/1/pkg-config) to locate dependencies but they way this is wired up is relying on an install directory and the original build system producing a `.pc` file with the appropriate info.

My initial thought is that it would be nice to have a `pkg_config_library` rule that wraps `cc_library` and consume the necessary info to fill generate a `.pc` file with all the same sanitizations the other `rules_foreign_cc` rules do to provide seamless inter-op.

But I'm curious what others think and if there are existing solutions.

Begin with the linked BCR pull request comment and the existing rules_foreign_cc/rules_cc integration around ffmpeg. Determine whether an existing pkg-config solution covers the use case; a satisfactory outcome should define the rule's scope and interoperability requirements before implementation, with validation for generated .pc metadata and cross-rule consumption.

Written by the indexing model from the issue text.