feat: add isEncrypted if set to false or true in notify command

[murali-shris]

Fixes https://github.com/atsign-foundation/at_libraries/issues/650 - What I did

• Set isEncrypted flag in notify command whether it is true or false - How I did it
• Temporarily introduce a private method _toAtProtocolFragment(duplicate of AtKey-->Metadata --> toAtProtocolFragment). Set isEncrypted whether it is true or false. Remove this private method and reuse AtKey method once update verb isEncrypted changes are done. - How to verify it
• tests in dependent repos should pass

https://github.com/atsign-foundation/at_libraries/pull/646 https://github.com/atsign-foundation/at_server/pull/2087 https://github.com/atsign-foundation/at_client_sdk/pull/1393 https://github.com/atsign-foundation/at_libraries/pull/647 https://github.com/atsign-foundation/noports/pull/1344

[murali-shris]

This is fine as a stopgap. As a precaution, please add some unit tests re isEncrypted when using an UpdateVerbBuilder

https://github.com/atsign-foundation/at_libraries/pull/649/commits/141716aa9586faf49cdc04f2c18ad5a5695e4c8c Are these tests fine ?

[murali-shris]

I will rerun the tests once below PR is merged and then merge at_commons PR https://github.com/atsign-foundation/at_client_sdk/pull/1391