ci: implement phase 1 path-based workflow skipping (#3332)

* ci: add phase 1 path-based workflow skipping

Signed-off-by: Georg Heiler <georg.kf.heiler@gmail.com>

* ci: add fast pull_request_target lint checks

Signed-off-by: Georg Heiler <georg.kf.heiler@gmail.com>

* ci: keep pr fast checks cheap

Signed-off-by: Georg Heiler <georg.kf.heiler@gmail.com>

* ci: expand full matrix triggers

Signed-off-by: Georg Heiler <georg.kf.heiler@gmail.com>

* ci: enable same-repo and merge queue checks

Signed-off-by: Georg Heiler <georg.kf.heiler@gmail.com>

* ci: harden pull_request_target fetch inputs

Signed-off-by: Georg Heiler <georg.kf.heiler@gmail.com>

* ci: address phase 1 workflow review

Signed-off-by: Georg Heiler <georg.kf.heiler@gmail.com>

* ci: grant reusable checks permissions

Signed-off-by: Georg Heiler <georg.kf.heiler@gmail.com>

* ci: temporarily enable pr fast checks validation

Signed-off-by: Georg Heiler <georg.kf.heiler@gmail.com>

* ci: allow first run of pr fast checks

Signed-off-by: Georg Heiler <georg.kf.heiler@gmail.com>

* ci: load pr fast check script for first validation

Signed-off-by: Georg Heiler <georg.kf.heiler@gmail.com>

* ci: format pr fast check script

Signed-off-by: Georg Heiler <georg.kf.heiler@gmail.com>

* ci: guard temporary pr fast check script fallback

Signed-off-by: Georg Heiler <georg.kf.heiler@gmail.com>

* ci: use pr metadata for temporary fast check validation

Signed-off-by: Georg Heiler <georg.kf.heiler@gmail.com>

* ci: remove temporary pr fast checks trigger

Signed-off-by: Georg Heiler <georg.kf.heiler@gmail.com>

* ci: disable duplicate pull request runs

Signed-off-by: Georg Heiler <georg.kf.heiler@gmail.com>

* ci: run fast pr checks without path trigger filter

Signed-off-by: Georg Heiler <georg.kf.heiler@gmail.com>

* ci: add job timeouts in checks.yml

Cap every job so a stuck runner cannot burn the 6h default. Limits:
changes=5, lint=20, run-tests-1/2=45, run-examples=60,
test-pip-install-*=30, build/test-package=15.

Signed-off-by: Georg Heiler <georg.kf.heiler@gmail.com>

* ci: restore pull request workflow triggers

Signed-off-by: Georg Heiler <georg.kf.heiler@gmail.com>

* ci: run lint on pull requests

Signed-off-by: Georg Heiler <georg.kf.heiler@gmail.com>

---------

Signed-off-by: Georg Heiler <georg.kf.heiler@gmail.com>