SDKs should get the token exchange endpoint from the issuer's configuration

[rhamzeh]

Originally reported by @stefanmilivojevic91 for the .NET SDK

Token endpoint path should be fetched from issuer's openid configuration (.well-known/openid-configuration) endpoint. AzureAD has oauth2/token for instance and OAuth2Client is unable to fetch access token using client credentials flow

• JS SDK
• Go SDK
• Python SDK
• .NET SDK: PathTemplate auth/token hardcoded in OAuth2Client ExchangeTokenAsync method dotnet-sdk#30
• Java SDK

[aaguiarz]

Instead of reading it from the OpenID discovery endpoint, we can let users configure it directly, to avoid querying the endpoint.

[rhamzeh]

Duplicated by a newer issue: #238, closing in favour of that