add support for validating boolean values

the transformValidationOption allows to convert string to type values using enableImplicitConversion option

const transformedValidatedValues = await transformAndValidate(
schema,
requestPayload,
{ transformer: { enableImplicitConversion } } // E.g. string numbers will be converted to type number
);
however there is one issue. if your schema specifies one boolean property and the request payload pass any string other than 'true' or 'false' the transformer converts it into true.

Start by tracing transformAndValidate and transformValidationOption when enableImplicitConversion is enabled. Reproduce the boolean conversion described in issue #46, then verify that strings other than 'true' or 'false' are not accepted as true while valid boolean values continue to work.

Written by the indexing model from the issue text.