chore: add PR approval gate for new contributors

- Add APPROVED_CONTRIBUTORS file with existing contributors
- Add pr-gate.yml workflow to close PRs from unapproved contributors
- Add approve-contributor.yml workflow to approve via lgtm on issues
- Add CONTRIBUTING.md with guidelines
- Update README.md to point to CONTRIBUTING.md

.github/APPROVED_CONTRIBUTORS

@@ -0,0 +1,84 @@
+# GitHub handles of users approved to submit PRs
+# One handle per line (without @)
+# Add new contributors by commenting lgtm on their issue
+aadishv
+airtonix
+aliou
+aos
+austinm911
+banteg
+ben-vargas
+butelo
+can1357
+CarlosGtrz
+cau1k
+cmf
+crcatala
+Cursivez
+cv
+dannote
+default-anton
+dnouri
+DronNick
+enisdenjo
+ferologics
+fightbulc
+ghoulr
+gnattu
+HACKE-RC
+hewliyang
+hjanuschka
+iamd3vil
+jblwilliams
+joshp123
+jsinge97
+justram
+kaofelix
+kiliman
+kim0
+lockmeister
+LukeFost
+lukele
+m-box-mr
+marckrenn
+markusylisiurunen
+mcinteerj
+melihmucuk
+mitsuhiko
+mrexodia
+nathyong
+nickseelert
+nicobailon
+ninlds
+ogulcancelik
+patrick-kidger
+paulbettner
+Perlence
+pjtf93
+prateekmedia
+prathamdby
+ribelo
+richardgill
+robinwander
+ronyrus
+roshanasingh4
+scutifer
+skuridin
+steipete
+svkozak
+tallshort
+theBucky
+thomasmhr
+tiagoefreitas
+timolins
+tmustier
+tudoroancea
+unexge
+vaayne
+VaclavSynacek
+vsabavat
+w-winter
+Whamp
+WismutHansen
+XesGaDeus
+yevhen