[BREAKING] - Refactor NimBLEAdvertising

* General code cleanup.
* Added new method, `NimBLEAdvertising::setPreferredConnectionInterval` that takes the min and max preferred connection parameters as an alternative for `setMinPreferred` and `setMaxPreferred`.
* Added new method, `NimBLEAdvertising::setAdvertisingInterval` Sets the advertisement interval for min and max to the same value instead of calling `setMinInterval` and `setMaxInterval` separately if there is not value difference.
* `NimBLEAdvertisementData` payload is now stored in `std::vector<uint8_t>` instead of `std::string`.
* `NimBLEAdvertisementData::getPayload` now returns `std::vector<uint8_t>` instead of `std::string`.
* `NimBLEAdvertisementData::addData` now takes either a `std::vector<uint8_t>` or `uint8_t* + length` instead of `std::string` or `char + length`.
* `NimBLEAdvertisementData::setName` now takes an optional `bool` parameter to indicate if the name is complete or incomplete, default = complete.
* `NimBLEAdvertising::start` No longer takes a callback pointer parameter, instead the new method `NimBLEAdvertising::setAdvertisingCompleteCallback` should be used.
* `NimBLEAdvertising::setAdvertisementType` has been renamed to `NimBLEAdvertising::setConnectableMode` to better reflect it's function.
* Added new method, `NimBLEAdvertising::setDiscoverableMode` to allow applications to control the discoverability of the advertiser.
* Advertising the name and TX power of the device will no longer happen by default and should be set manually by the application.

Author: h2zero