uploads: Skip the outgoing proxy if S3_KEY is unset.

When the credentials are provided by dint of being run on an EC2
instance with an assigned Role, we must be able to fetch the instance
metadata from IMDS -- which is precisely the type of internal-IP
request that Smokescreen denies.

While botocore supports a `proxies` argument to the `Config` object,
this is not actually respected when making the IMDS queries; only the
environment variables are read from.  See
https://github.com/boto/botocore/issues/2644

As such, implement S3_SKIP_PROXY by monkey-patching the
`botocore.utils.should_bypass_proxies` function, to allow requests to
IMDS to be made without Smokescreen impeding them.

Fixes #20715.

zerver/lib/upload.py

@@ -85,6 +85,14 @@ INLINE_MIME_TYPES = [
 # through a sanitization function.
 
 
+# https://github.com/boto/botocore/issues/2644 means that the IMDS
+# request _always_ pulls from the environment.  Monkey-patch the
+# `should_bypass_proxies` function if we need to skip them, based
+# on S3_SKIP_PROXY.
+if settings.S3_SKIP_PROXY is True:  # nocoverage
+    botocore.utils.should_bypass_proxies = lambda url: True
+
+
 class RealmUploadQuotaError(JsonableError):
     code = ErrorCode.REALM_UPLOAD_QUOTA