fix: get buck2 working in OSS clone

[ben--]

Create the necessary buck config. Most of the config is adapted from folly's external configuration.

Test

This diff enables the following command to run:

~/oss/sapling$ buck2 targets eden/fs/service:
Build ID: 8f032f3a-d73a-424d-acca-7d85d340bd93
Command: targets.
Time elapsed: 0.0s
gh_facebook_sapling//eden/fs/service:edenfs
gh_facebook_sapling//eden/fs/service:edenfs-oss
gh_facebook_sapling//eden/fs/service:edenfs_privhelper
gh_facebook_sapling//eden/fs/service:edenfs_privhelper_with_setuid_root
gh_facebook_sapling//eden/fs/service:init
gh_facebook_sapling//eden/fs/service:java-swift-java11-thrift-dependencies
gh_facebook_sapling//eden/fs/service:main
gh_facebook_sapling//eden/fs/service:pretty_printers
gh_facebook_sapling//eden/fs/service:server
gh_facebook_sapling//eden/fs/service:startup_logger
gh_facebook_sapling//eden/fs/service:startup_status_subscriber
gh_facebook_sapling//eden/fs/service:thrift_get_object_impl
gh_facebook_sapling//eden/fs/service:thrift_glob_impl
gh_facebook_sapling//eden/fs/service:thrift_startup_status_subscriber
gh_facebook_sapling//eden/fs/service:thrift_util
gh_facebook_sapling//eden/fs/service:usage_service
~/oss/sapling$