Split JWT validation API for 2 APIs

[ldclakmal]

Description:
The JWT validation API need to be splitted into 2 for public and internal usages as follows:

public isolated function validate(string jwt, ValidatorConfig validatorConfig) returns Payload|Error { }

isolated function validateJwt(string jwt, ValidatorConfig validatorConfig, cache:Cache? jwksCache = ()) returns Payload|Error { }