fix: added a check to handle self symmetric shared

[Xlin123]

- What I did
I fixed bug with AtClient.get and shared symmetric keys #1071
- How I did it
Added symmetric shared key specific exceptions and also made sure they used proper encryption type on the keys.
- How to verify it
A made a test in decryption_service_test, I'm not sure if I did it right or if it even ran in the suite.
- Description for the changelog
Fixed bug with AtClient.get and shared symmetric keys

[Xlin123]

conclusion: to fix the issue I need to use at_chops to do RSA encryption because it's trying to use the private symmetric key to decrypt an AES key. If there's any place where at_chops is used so I can see an example it would be very helpful.

[gkc]

See here to understand what happens when, as @bob, you decrypt a message from @alice https://github.com/atsign-foundation/at_protocol/blob/trunk/usage-examples/how-to-exchange-encrypted-data.md#4-fetch-the-symmetric-key-which-alice-shared

The code which handles this is in shared_key_decryption.dart, and part of that code handles the fetching of the symmetric key, decoding from base64, and decrypting using at_chops and the 'receiving' atSign's private key

[Xlin123]

Ahhh, okay got it. I did some work on self_key_decryption first since keys like "shared_key.bob@alice" I think fall under self keys. Let me know if that's incorrect and I can just revert changes. I'll do shared_key_decryption when I get home.

[gkc]

The copy alice will create for themselves of a shared key they cut for bob will be shared_key.bob@alice. Will this test work? What happens when you use this branch with your REPL?

[gkc]

sharedWith is not set - when we create shared_key.bob@alice - again, what happens when you run this branch with your REPL? Do you get the desired outcomes?

[Xlin123]

Two decryptable cases

However, /get @bob:shared_key@alice isn't throwing it's exception properly. I should finish it tonight.

[Xlin123]

@gkc I think I fixed everything.

[gkc]

Please add unit tests which verify each case