JWT Builder shortcuts

[sberyozkin]

This PR continues with making the builder API as compact as possible, introduces the shortcuts to optimize some flows.

1. In the flow where the claims are built from the empty builder, Jwt.claims() transition is now optional, every time I see Jwt.claims().claim("a", "b") I think this is suboptimal.
   So, instead of Jwt.claims().issuer("issuer").subject("sub").sign() one can just do Jwt.issuer("issuer").subject("sub").sign().

2. Creates sign(), encrypt and innerSignAndEncrypt shortcuts to optimize the flows where the keys are configured and where the default RSA algorithms are used:

• Jwt.claims("/token.json").sign() is equivalent to Jwt.sign("/token.json")
  (same for the maps and jsonobject)
• Jwt.claims("/token.json").jwe().encrypt() is equivalent to Jwt.encrypt("/token.json")
  (same for the maps and jsonobject)
• Jwt.claims("/token.json").innerSign().encrypt() is equivalent to Jwt.innerSignAndEncrypt("/token.json")
  (same for the maps and jsonobject)

3. Also added Instant overloads for the iss and exp claims

[MikeEdgar]

Looks good to me

[sberyozkin]

@MikeEdgar @radcortez thanks