fix: fix serverless proxy signing (#2101)
fix: fix serverless proxy signing (#2101)

Fixes 2 issues with the signing:
1. We are not prepending the `sha1=`
2. We are not using the correct secret value. The secret from Secret Manager is actually a JSON blob and the secret is one value from it that we need to parse out. (previously, we were using the entire secrets blob as the secret)
3. The signatures should be hex encoded, not base64

Fixes #2092

googleapis/repo-automation-bots,googleapis/repo-automation-bots,googleapis/repo-automation-bots,googleapis/repo-automation-bots,googleapis/repo-automation-bots