Enforce encryption for a notify characteristic

The current version does not support enforcing encryption when configuring a notify characteristic.
This is something I need for my project and so I added it to get things working.

I think this is a feature that others might like, so attached is a diff of the files that I changed for my project.
Although, this code doesn't distinguish between notify and indicate encryption since there was only 1 bit free in the characteristic flags field.
So you probably can't use the patch directly.

[notify_encryption.txt](https://github.com/apache/mynewt-nimble/files/9576579/notify_encryption.txt)

Start by reviewing the attached notify_encryption.txt diff and the characteristic flag handling it changes. Determine how encryption should be represented for notify characteristics given the single remaining flag bit, then verify that configuring a notify characteristic can enforce encryption without incorrectly conflating notify and indicate behavior.

Written by the indexing model from the issue text.