all: add support for @BUILD_DATE, @BUILD_TIME and @BUILD_TIMESTAMP, all using v.util.get_build_time(), and overridable through SOURCE_DATE_EPOCH

[spytheman]

With this PR, this code:

dump(@BUILD_DATE)
dump(@BUILD_TIME)
dump(@BUILD_TIMESTAMP)

will produce something like this (fixed at compile time), each time it is run:

[/v/misc/2024_09_13__12/a.v:2] @BUILD_DATE: 2024-09-13
[/v/misc/2024_09_13__12/a.v:3] @BUILD_TIME: 09:31:25
[/v/misc/2024_09_13__12/a.v:4] @BUILD_TIMESTAMP: 1726219885

... which eliminates one more common use of a separate build system.

[koplenov]

I think we still need a git commit hash

[koplenov]

Hmm, I did it this way

@["/utils/get/server/version"]
pub fn (mut server Server) utils_get_server_version(mut ctx Context) veb.Result {
	return ctx.json[map[string]string](
		{
			"commit": $embed_file('.git/refs/heads/master', .zlib).to_string()
			"build_date": @BUILD_DATE
			"build_time": @BUILD_TIME
			"build_timestamp": @BUILD_TIMESTAMP
		}
	)
}