API parse errors

[sgrimee]

Using this issue to collect API deserialization errors due to missing fields.

• The field url in the Activity struct is not always sent by the API, causing a parse error on occasion.
  This can be seen for example when receiving a Meeting event on meeting closure.

• The field global_id can be missing to build a Target struct. This was seen when receiving a delete (?) activity

These are minor as long as the crate does not support the associated event types, but it would be cleaner to make the fields optional.

[sgrimee]

global_id was made optional in Target in commit 4b701de

[sgrimee]

participants and activities now optional in Target in commit efd4935

[sgrimee]

seen with AdaptativeCards

Msg(
        "Failed to list webex::types::Message",
    ),
    State {
        next_error: Some(
            Error(
                Msg(
                    "failed to parse reply",
                ),
                State {
                    next_error: Some(
                        Error("unknown variant `stretch`, expected `Auto` or `Stretch`", line: 1, column: 924),
                    ),
                    backtrace: InternalBacktrace {
                        backtrace: None,
                    },
...

[sgrimee]

seen with AdaptativeCards

    Msg(
        "Failed to list webex::types::Message",
...
                        Error("missing field `items`", line: 1, column: 795),

[sgrimee]

                    Error("missing field `items`", line: 1, column: 795),

fixed in d944c31

[sgrimee]

                    Error("unknown variant `stretch`, expected `Auto` or `Stretch`", line: 1, column: 924),

fixed in 8df385a