Adds support for the SDK to automatically modify the HTTP client
to include TLS configuration of custom Client TLS certificate. This
configuration can be provide via the environment variable or directly in
code via the `session.Options` struct.

These options are compatible with the AWS_CA_BUNDLE configuration.

Environment variable configuration

Both `AWS_SDK_GO_CLIENT_TLS_CERT`, and `AWS_SDK_GO_CLIENT_TLS_KEY` must
be provided together, and must point to valid PEM encoded file
containing the certificate, and key respectively.

   AWS_SDK_GO_CLIENT_TLS_CERT=$HOME/my_client_cert
   AWS_SDK_GO_CLIENT_TLS_KEY=$HOME/my_client_key

In code configuration via session.Options

Alternative configuration is to specify the `ClientTLSCert` and
`ClientTLSKey` fields on the `session.Options` struction. These are
`io.Reader`s that provide the PEM encoded content for the certificate
and key files.

  sess, err := session.NewSessionWithOptions(session.Options{
      ClientTLSCert: myCertFile,
      ClientTLSKey: myKeyFile,
  })