Add public Jwt type

[giarc3]

Switched from frank_jwt to jsonwebtoken.

There already was an internal Jwt type, so re-purposed it and made it public. It validates that the jwt has the correct payload fields and an allowed algorithm. user_create, user_verify, and generate_new_device all use the new Jwt type instead of &str.

Also fixes:

• Windows can't run integration tests with tls-rustls or tls-default features #162
• Replace dev depencency frank_jwt with jsonwebtoken #163

[clintfred]

Looks good.

Note that with this change this is the first time we are using Windows' native TLS implementation to talk to the web service.