* add comments detailing logic, migrate middleware::custom::tests to frontend::custom
* simplify custom predicate's frontend interfaces, making it less verbose to define Statement Template arguments
The main idea is that when defining the arguments at a statement
template, it can be done from 3 different inputs:
i. `(&str, literal)`: this is to set a POD and a field, ie. `(POD, literal("field")`)
ii. `(&str, &str)`: this is to define a origin-key wildcard pair, ie. `(src_origin, src_dest)`
iii. `Value`: this is to define a literal value, ie. `0`
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