federator: Expect a client certificate to be the certificate chain

[akshaymankar]

Without this openssl doesn't forward to whole chain causing mTLS to not succeed.

Backport of #4089

https://wearezeta.atlassian.net/browse/WPB-9692

Checklist

• [x] Add a new entry in an appropriate subdirectory of changelog.d
• [x] Read and follow the PR guidelines