support authentication methods other than username/password

[rursprung]

the first version (#25) only introduces username/password authentication (easiest to implement & test), however opensearch offers various other authentication options (see the documentation) and we should support all of them eventually.

this issue should be used to define how this should be implemented and either do it directly for all of them (unlikely) or raise follow-up issues for the different authentication options to be implemented later on (with a clear way forward for them).

[reta]

At the moment, we provide HttpClientConfigCallback / HttpAsyncClientBuilder to allow configuring any aspect of the underlying client(s), including other authentication methods if supported. My concern regarding baking everything into the client itself is that it may not be practical - there are tons of different LDAP / SAML / OpenID / OAuth / JWT / ... libraries there, we cannot support all nor force to use one.