ref: Add rust arroyo, second attempt

[untitaker]

(reapplying #323 because I accidentally squash-merged)

Move ./rust_snuba/rust_arroyo/ from snuba into arroyo, preserving history.

Steps used to preserve history:

git remote add snuba git@github.com:getsentry/snuba
git checkout -b ref/add-rust-arroyo
git reset --hard snuba/master
git filter-branch --subdirectory-filter rust_snuba/rust_arroyo/ -- --all
mkdir rust-arroyo
mv Cargo.toml examples/ src/ rust-arroyo/
git add ...
git commit -am 'move arroyo to subdirectory'
git merge --allow-unrelated-histories main

maybe we should not squash-merge this one? unsure.

this PR doesn't contain any other code changes. followup PRs include:

• https://github.com/getsentry/arroyo/pull/324