feat: support ssl cert from string

[Chaz6]

This PR does the following:-

• Adds a new optional attribute called ca_cert to keycloak_openid.KeycloakOpenID
• Adds a new optional attribute called ca_cert to connection.ConnectionManager
• Adds a private class called _HTTPAdapterWithSslContext to connection

The result is that it is possible to supply the CA cert data as a string. This is useful when this data may be stored somewhere other than a local file, such as an external network service.