Improve eventlogmessage stage in loki.process

[wildum]

Request

As described in #982 and grafana/loki#9648, the parser of the eventlogmessage stage should be improved to support:

• lines that don't have the key:value format
• values that are spread across several lines
• trim the keys to make it easier to use in later stages (currently \t\tSecurity ID is extracted as __Security_ID)

The changes should either be done in a non-breaking way or be planned for 2.0