Add ClientAuthenticationMethod#toString

[jzheaux]

The error message:

This class supports client_secret_basic, client_secret_post, and none by default. Client [%s] is using [%s] instead. Please use a supported client authentication method, or use setRequestEntityConverter to supply an instance that supports [%s].

renders like this:

This class supports client_secret_basic, client_secret_post, and none by default. Client [salesforce-client] is using [org.springframework.security.oauth2.core.ClientAuthenticationMethod@3b4abc2b] instead. Please use a supported client authentication method, or use setRequestEntityConverter to supply an instance that supports [org.springframework.security.oauth2.core.ClientAuthenticationMethod@3b4abc2b]

Mistakes like this would be less common if ClientAuthenticationMethod had a reasonable toString implementation.

Related to #13476 and #13496