Add coercion functions to the remap syntax

[binarylogic]

A common use for the new remap syntax will be the need to coerce log fields to different types. This helps with the generic processing of log data and ensuring a consistent schema. It should deprecate the coercer transform.

Usage

All of these functions should be used on the right side of the = when evaluating:

.message = string(.message)
.status = int(.status)

Requirements

• Add a string(.path.to.field) function
• Add a int(.path.to.field) function
• Add a float(.path.to.field) function
• Add a bool(.path.to.field) function
• Add a timestamp(.path.to.field, "%F") function

[binarylogic]

This appears to have already been covered in #3341.