Remap support for the `filter` transform

The filter transform for Vector enables you to winnow down a stream of events to only those that match a specified condition.

Previously, the filter transform required you to specify conditions using check_fields. But now you can use Vector Remap Language expressions in the remap transform to specify those conditions. This should make using filter more natural and expressive.

#Example

The example configuration below shows the same filter transform using the old system (check_fields) and the new system (remap):

[transforms.filter_out_non_critical]
type = "filter"
inputs = ["http-server-logs"]f

# Using check_fields
condition.type = "check_fields"
condition.message.status_code.ne = 200
condition.message.severity.ne = "info"
condition.message.severity.ne = "debug"

# Using remap
condition.type = "remap"
condition.source = '.status_code != 200 && !includes(["info", "debug"], .severity)'