Make core capabilities optional on accounts

[korrat]

The spec does not mandate the presence of an "urn:ietf:params:jmap:core" capability on the account objects in the JMAP session. The example does not include this capability either. Additionally, the current implementation does not use these capabilities and only specifies an empty object.

Therefore, this patch makes the properties optional. This means, mujmap can synchronize with a Stalwart mail server.

[sanderhollaar]

Should PrimaryAccounts at https://github.com/elizagamedev/mujmap/blob/main/src/jmap/session.rs#L41 also be patched?

After applying your patch I still got Failed to read JSON: missing field urn:ietf:params:jmap:core at line 1 column 3952.

I failed to come up with a similar patch so I ended up commenting the lines:

$ grep -nA2 'pub struct PrimaryAccounts' src/jmap/session.rs
41:pub struct PrimaryAccounts {
42-//    #[serde(rename = "urn:ietf:params:jmap:core")]
43-//    pub core: Id,