Throttle filter add template support

[Kokan]

This PR adds template supports, and as a last step removes value() option.
value("A") can be replaced with template("${A}").

This PR somewhat conflicts with:

• news: add entry for #3781 #3816 (news entry)
• Light: add testcase for throttle() filter #3815 (light tests)

I'll provide patches in those, but as the result no separate news entry is needed here.

Example config:

   throttle(
     template("${.journald._UID}")
     rate(1)
   );

[kira-syslogng]

Build SUCCESS

[OverOrion]

LGTM, I have tried it out, it works as intended.

[Barkodcz]

I found a crash :/

If I didn't set in the filter the template option.

Example config

source s_network {
	network(
		ip("0.0.0.0")
		port(6666)
	);
};

destination d_network {
	network(
		"127.0.0.1"
		port(6667)
	);
};

filter f_throttle {
	throttle(
		# template("") <-- I didn't set 
		rate(1)
	);
};

log {
	source(s_network);
	filter(f_throttle);
	destination(d_network);
};

[Kokan]

Nice catch, the last commit also removed the check if the template was provided (77d2881).

[kira-syslogng]

Build SUCCESS

[Barkodcz]

LGTM, approve.

[Kokan]

Only rebased on current master to make CI happy.

[kira-syslogng]

Build SUCCESS

[OverOrion]

LGTM, approved!