feat: Support forwarding key detection and filtering

- Add IsForwardingKey method to identify forwarding keys
- Add NonForwardingKeys to filter out forwarding keys from keyrings

chore: Prepare release

ci: Adapt CI to proton branch

Merge pull request ProtonMail#343 from ProtonMail/release/gopenpgp-v2…

….9.0

Prepare release v2.9.0, and update CI/Lints.

ci: Adapt to new go version

chore: Prepare release 3.2.1

ci: Adapt CI to proton branch

chore: Prepare release 3.2.0

ci: Adapt CI to proton branch

chore: Prepare release 3.1.3 (ProtonMail#327)