CONTEXT: lazily evaluate frames to only actually assemble when needed

[Al2Klimov]

[2022-10-28 17:38:10 +0200] information/cli: Icinga application loader (version: v2.13.0-461-g8e3a13020; debug)
[2022-10-28 17:38:10 +0200] information/cli: Loading configuration file(s).
[2022-10-28 17:38:11 +0200] information/ConfigItem: Committing config item(s).
[2022-10-28 17:38:11 +0200] critical/config: Error: Error while evaluating expression: Tried to access undefined script variable 'x'
Location: in /Users/aklimov/NET/WS/icinga2/prefix/etc/icinga2/conf.d/services.conf: 31:53-31:53
/Users/aklimov/NET/WS/icinga2/prefix/etc/icinga2/conf.d/services.conf(29):   check_command = "ping4"
/Users/aklimov/NET/WS/icinga2/prefix/etc/icinga2/conf.d/services.conf(30):
/Users/aklimov/NET/WS/icinga2/prefix/etc/icinga2/conf.d/services.conf(31):   assign where host.address && ((function() { throw x })())
                                                                                                                               ^
/Users/aklimov/NET/WS/icinga2/prefix/etc/icinga2/conf.d/services.conf(32): }
/Users/aklimov/NET/WS/icinga2/prefix/etc/icinga2/conf.d/services.conf(33):

Context:

	(0) Evaluating 'apply' rule (in /Users/aklimov/NET/WS/icinga2/prefix/etc/icinga2/conf.d/services.conf: 26:1-26:21)
	(1) Evaluating 'apply' rules for host 'alexandersmbp2.int.netways.de'
[2022-10-28 17:38:11 +0200] critical/config: 1 error
[2022-10-28 17:38:11 +0200] critical/cli: Config validation failed. Re-run with 'icinga2 daemon -C' after fixing the config.

closes #9556

[julianbrost]

You have already changed some callers of CONTEXT(). There are quite a few that use the + operator for building the string. It might make sense to consistently use one or the other (less from a performance point of view but more from a "this is the intended way to use this" point of view).