Set correct ClientCredentialStyle for use with ClientAssertion

[kbrekke]

The bug can be reproduced by running https://github.com/IdentityModel/IdentityModel.OidcClient/tree/main/clients/ConsoleClientWithBrowser

[josephdecock]

Thanks very much for catching this @kbrekke! I've updated this to conditionally set the client credential style, because if we are using client secrets instead of client assertions, we shouldn't set them in the post body (doing so is strongly discouraged by RFC 6749).

[josephdecock]

@brockallen, I updated this PR, so would you mind taking a quick look as well? My thinking is that, because client secrets are not recommended in the post body by RFC 6749, and RFC 9126 (PAR) says that

The rules for client authentication as defined in [RFC6749] for token endpoint requests, including the applicable authentication methods, apply for the PAR endpoint as well.

we should use the post body credentials style (at least by default) only for client assertions. Do you agree?