Envoy-data-plane

Updated master branch to envoy 1.21.0 protobufs

* Added prometheus protobufs
* Script now compiles all protobufs at once, instead of incrementally, which should result in less inconsistency
* Converted the project to use github actions instead of travis

Bumped betterproto to 2.0.0b3 which should allow creating a grpc server using service base classes

Few fixes, better download script for protos, corrected a test.

May try to work out in future how to release frozen versions of past envoy protobufs

Library now uses `betterproto==2.0.0b2` which should fix a few issues where fields don't appear in JSON representations because they are empty. For example: https://github.com/cetanu/envoy_data_plane/commit/26dde008f68f8c1a092b5f1425783df1eb0e9025#diff-b3af45c25c1b1a9b6e3b29ed920e5aa228521116445274f529f5b5f36811e4e9

The build process is also much more consistent on my Windows machine, so hopefully I can make more improvements, without pain!