Fix decoding error triggered by Assistants API responses with data from searching a vector database

[mplawley]

When the Assistants API is asked to search a file store, it replies with a response that does not include properties required by the MessageObject's CodingKeys. This leads to a decoding error. This PR makes those values optional so that:

1. Current Assistants API responses that are successfully decoded are still decoded successfully
2. Assistants API responses with data from a file_search, which do not contain FileCitation.type, FileCitation.text, FileCitation.startIndex, and FileCitation.endIndex, also work

TEST PLAN

1. Create an Assistant AI in OpenAI and note its agent_id. Attach a file store with at least one text file.
2. Create a skeleton project that utilizes SwiftOpenAI's current main branch to handle an Assistant interaction, such as in a conversation.
3. Create a thread with the assistant and send a message, such as "May I ask you a question?"
4. Note that the response is decoded successfully to an in-memory representation of the OpenAI Response.
5. Now send a message such as "Please search your file store for advice on how to decode API responses."
6. Note the CodingKeys error.
7. Now switch to this branch. Perform steps 3-5. Note there are no errors and that the client can now receive responses that contain file citations from the Assistant.

[jamesrochabrun]

LGTM! thanks