This commit adds a new STS API for X.509 certificate
authentication.
A client can make an HTTP POST request over a TLS connection
and MinIO will verify the provided client certificate, map it to an
S3 policy and return temp. S3 credentials to the client.
So, this STS API allows clients to authenticate with X.509
certificates over TLS and obtain temp. S3 credentials.
For more details and examples refer to the docs/sts/tls.md
documentation.
Signed-off-by: Andreas Auernhammer <hi@aead.dev>