Add set_expires_in and set_issued_now methods

[bugdea1er]

Hi there!

I added set_expires_in and set_issued_now method for the jwt builder.

• set_issued_now sets a "iat" claim by calling system_clock::now() by itself;
• set_expires_in sets a "exp" claim by adding the given duration to the "iat" claim system_clock::now()

Common usage of the jwt builder is:

• create a builder
• set the "iat" claim at the "now" moment
• set the token expiration timeout

This should make it a little bit more convenient.

Update: builder now has the Clock template parameter just like the verifier

[prince-chrismc]

I think these are good QOL improvements. Just all of the examples

	auto token = jwt::create()
		.set_issuer("auth0")
		.set_type("JWT")
		.set_id("rsa-create-example")
-		.set_issued_at(std::chrono::system_clock::now())
-		.set_expires_at(std::chrono::system_clock::now() + std::chrono::seconds{36000})
+		.set_issued_now()
+		.set_expires_in(std::chrono::seconds{36000})
		.set_payload_claim("sample", jwt::claim(std::string{"test"}))
		.sign(jwt::algorithm::rs256("", rsa_priv_key, "", ""));

I would love to bring the verifier and builder closer in API design by making them use the default_clock and this way under the hood both of them can have a clock::now() to help them.

I think it makes sense the build could do the same as

jwt-cpp/include/jwt-cpp/jwt.h

Line 3478 in 953aab6

verify_ops::verify_context<json_traits> ctx{clock.now(), jwt, default_leeway};

WDYT?

[bugdea1er]

Should I update examples or README with the new methods?

[prince-chrismc]

Should I update examples or README with the new methods?

I think that would be excellent. Going over the code in more details, I think this new API is less error prone (prevents creating tokens that are already expired) and I'd love to see this being used going forward

[prince-chrismc]

Thanks for this awesome improvement!