fix: support json token file format authentication

[mattisonchao]

Motivation

Support JSON token file format authentication for Pulsar Admin.
E.g:

{"file": "./mnt/token"}

Modifications

• Add token file format for json.

Verifying this change

• Make sure that the change passes the CI checks.

[BewareMyPower]

Is it necessary? This could be done from the application side.

It looks unnecessarily complicated because now File has two supported format <token> or {"file": "<token>"}.

The existing NewAuthenticationTokenFromFile has poor design that should be deprecated.

[mattisonchao]

Hi, @BewareMyPower

We should use JSON format as a primary format to configure the token with different key-value pairs. That will be more flexible.

E,g:

• {"token": ""}
• {"file": ""}

But this repo has two components.

• client
• admin

The JSON format only works for a client, but not for the admin.

[BewareMyPower]

It makes sense to use the same way to configure JWT authentication for admin and client.

But from the perspective of designing the API from the very beginning:

1. One format is enough. Pure token string is enough, JSON is scalable but it's over-designed. At least the server side only processes the token string.
2. Reading from file (or any other external data source like DB or even a topic) is unnecessary. It should be done from the application side.