Make Client Assertions more extensible

[brockallen]

For this property: https://github.com/IdentityModel/IdentityModel.OidcClient/blob/main/src/OidcClient/OidcClientOptions.cs#L59

This value might change during use (e.g. 60 second expiration), so we want something more extensible to allow for dynamic access to a client assertion. Just an event on the options would be sufficient.

So now we could call the new event, and if it doesn't produce a value fall back to the static config value.