app/vlinsert: splunk support

[AndrewChubatiuk]

Describe Your Changes

• replaced fastjson.Parser with fastjson.Scanner, since one line can contains multiple log entries
• added separate endpoint for splunk, but most likely more customizations are needed
• added CORS headers, which are required for frontend, that is sending events to splunk API, but maybe it should be done on vmauth level

TODO:

• provide an option how to set stream fields. possibly they can be set from .fields field mapping
• provide an option how to set _msg field, since it's not possible to set custom headers or query args and .event field can contain json data. most straightforward solution is keep event field as it is, but obviously compression is less efficient in this case

Checklist

The following checks are mandatory:

• My change adheres to VictoriaMetrics contributing guidelines.
• My change adheres to VictoriaMetrics development goals.