Support for a custom json encoder

[ahmadnazir]

The current implementation relies on the native php functions for encoding/decoding json. The flags provided are JSON_UNESCAPED_SLASHES.

Depending on who creates the access token, the json encoding rules might be different. I have a case where I am getting an access token from Microsoft and the special characters are not encoded in json strings, and hence I have to use JSON_UNESCAPED_UNICODE as well.

This commit supports injecting a custom Json encoder.

Then main changes are:

• All places that refer to encoder have been renamed to base64Encoder
• Methods setEncoder are renamed to setBase64Encoder
• namespace \Namshi\JOSE\Base64 is moved to \Namshi\JOSE\Encoder to support json encoders along with other encoders
• The default JsonEncoder class is added

[odino]

Hi @ahmadnazir,

I just added a deprecation notice to this repo: since we're not using it anymore, it's very hard for us to keep developing namshi/jose. Happy to give it out to other maintainers if anyone wants to give it a shot! :)

At the same time, I'd recommend migrating to firebase/php-jwt as it seems to be here to stay :)