[ENHANCEMENT] - ability to change credentials (type) for existing client

[raybog]

Is your feature request related to a problem? Please describe.
Credentials are static and set when connection/client to ES is created.

Describe the solution you'd like
It should be possible to change the Credentials on each API call e.g.:
some_connection.transport().setCredentials(Credentials::Bearer(some_token)).get(GetParts::IndexId(&INDEX, &id)).pretty(true).send().await?

Describe alternatives you've considered
Creating a new connection on each request when different Credentials has to be used.