Allow custom registrations for built-in ops in MicroMutableOpResolver. (#3178)

* Allow custom registrations for built-in ops in MicroMutableOpResolver.

This change updates most `Add<OpName>()` methods in `MicroMutableOpResolver` to accept an optional `const TFLMRegistration& registration` argument. This enables users to provide a custom implementation for a built-in operator, while still defaulting to the standard registration if no argument is provided.

* Fix style

* Fix whitespace

Author: jerryjzy