Add options to set headers when initializing client

[htnguyen7]

To be able to use additional security

https://docs.expo.dev/push-notifications/sending-notifications/#additional-security

[ameyerfr]

@pablonahuelgomez can you please give us feedback on this ? thanks

[uxxman]

+1 needed.

Also, since the node library for expo-server-sdk supports it, all other languages can also add it.

[JacobGalati]

Can we get any movement on this? It seems silly that it isn't supported.