tendermint: Add now_utc method to time::Time

[thanethomson]

Version(s) of tendermint-rs: v0.23.x, master

Description

From informalsystems/hermes#1665, it's apparent that we should probably provide a mechanism in the time::Time struct in the tendermint crate to construct the current time in UTC, so we can avoid having to import the time crate to do that and then convert it to a time::Time struct.

A drawback of this, as pointed out by @mzabaluev, is that it needs to be feature-guarded since it requires std.

Definition of "done"

When we have a feature-guarded now_utc method on tendermint::time::Time similar to that which the time crate provides.

[seanchen1991]

It's not clear to me how the proposed now_utc method would defer from the tendermint::time::Time::now method:

tendermint-rs/tendermint/src/time.rs

Lines 68 to 70 in cd3e391

	pub fn now() -> Time {
	OffsetDateTime::now_utc().try_into().unwrap()
	}