fix r2 object array handling

[good-lly]

Describe the bug
When wrangler.toml R2 binding is done via array of objects, get R2 object constructor is not working properly...
also errors when instance is not properly configured/init are a bit annoying

To Reproduce
[[r2_buckets]]

this is working ->

binding = "MY_PROXY_BUCKET"

bucket_name = "openproxy-bucket"

this is NOT ->

r2_buckets = [
{ binding = "MY_PROXY_BUCKET", bucket_name = "openproxy-bucket", preview_bucket_name = "openproxy-bucket" }
]

[good-lly]

nei, focus to rewrite for s3 compatibility -> #13