feat(all): add methods to read/write descriptors

[trik]

Reading / writing descriptors could be useful when developing custom BLE services / characteristic.

In this commit:

• expose descriptors uuids in getServices return value
• add methods to read and write a descriptor value

[pwespi]

Thank you so much! Great work!

I changed a few small things:

• handle characteristics that don't have descriptors on web
• fix ios by adding the methods to Plugin.m and add descriptor discovery

[pwespi]

@all-contributors please add @trik for code

[allcontributors]

@pwespi

I've put up a pull request to add @trik! 🎉