Check Remap script result value type at compile-time

[JeanMertz]

The Remap language returns its last expression as a value when executed. This value can be a string, bool, integer, etc.

When using Remap in the context of a so-called "condition" within Vector, such as:

[transforms.reduce]
	type = "reduce"
	ends_when.type = "remap"
	ends_when.source = "<source code here>"

The condition expects to receive a boolean, which determines if reduced logs are emitted.

Currently, any non-boolean value in a remap-conditional is considered to be "false". For example:

ends_when.source = 'upcase("foo")' # is interpreted by the conditional logic as "false"

Goal

The goal of this RFC is to make the above example a compile-time error. The user is forced to explicitly return a boolean value in a conditional, for example by using the to_bool function:

ends_when.source = 'to_bool(upcase("foo"), default = false)'

This should not apply to the remap transform, as its return value is ignored (its expressions are used to manipulate the event itself, not to return a final script value). The remap transform scripts should thus be allowed to return any value.