Extend support of .regsub / .iregsub on build-in and user macros

Currently, these functions are supported only in following macros:

• {ITEM.VALUE}
• {ITEM.LASTVALUE}
• Low-level discovery macros (except in low-level discovery rule filter)

Please consider extending list of supported internal macroses.
Use cases are to extract some values from the macro and use them as arguments in scripts. Example - FQDN.

List of suggested macroses below, but any of supported ones could help in some specific configuration.:

{HOST.HOST(NAME)}
{HOST.DNS(CONN)}
{HOST.DESCRIPTION} {ITEM.NAME}
{ITEM.DESCRIPTION} {TRIGGER.NAME}
{TRIGGER.DESCRIPTION}