Fix parsing failing due to new TaskBacktrace activity log attachment

[fortmarek]

I'm not sure when TaskBacktrace was introduced but it seems to be a new of the section attachment. Example value from after tokenization:

[type: list, count: 2]
[type: classNameRef, className: "IDEFoundation.IDEActivityLogSectionAttachment"]
[type: string, value: "com.apple.dt.ActivityLogSectionAttachment.TaskBacktrace"]
[type: int, value: 1]
[type: int, value: 0]
[type: json, value: [{"description":"'Planning Swift module ConcurrencyExtras (arm64)' had never run","category":{"ruleNeverBuilt":{}},"identifier":{"storage":{"task":{"_0":[0,80,50,58,116,97,114,103,101,116,45,67,111,110,99,117,114,114,101,110,99,121,69,120,116,114,97,115,45,101,102,52,50,51,48,52,53,57,52,98,102,56,53,50,102,52,51,56,101,102,55,99,51,97,49,51,54,98,50,99,57,48,100,102,56,55,49,56,97,102,50,98,57,100,51,97,97,99,48,100,48,100,99,97,50,50,98,52,99,50,57,99,50,45,58,66,101,116,97,32,68,101,98,117,103,58,51,99,57,97,99,57,53,50,98,52,99,56,49,100,57,99,99,49,55,100,49,97,102,52,55,49,97,48,52,53,101,56]}}},"frameKind":{"genericTask":{}}}]]
[type: classNameRef, className: "IDEFoundation.IDEActivityLogSectionAttachment"]
[type: string, value: "com.apple.dt.ActivityLogSectionAttachment.TaskMetrics"]
[type: int, value: 1]
[type: int, value: 0]
[type: json, value: {"utime":15551,"stime":15551,"maxRSS":0,"wcStartTime":771519075106417,"wcDuration":15551}]

The second attachment is the one we currently parse, support for which was introduced in #204.

The first attachment is new – and currently XCLogParser fails when parsing it because it expects all attachments to be a task metric.

This PR adds support for parsing it. The JSON value is quite odd – I decided to parse just the category and description, discarding the identifier and frameKind. If you feel strongly, I can try adding support for those as well, but I'm not sure how they would be useful.

[fortmarek]

The backtrace logs only appear when the following setting is on: https://github.com/swiftlang/swift-build/blob/ab993847ee362913fe64f5d58a02770999c4d724/SwiftBuild.docc/Development/build-debugging.md?plain=1#L21

I've also realized, now that swift-build is open source, we can better structure the models based on the actual underlying source code: https://github.com/swiftlang/swift-build/blob/ab993847ee362913fe64f5d58a02770999c4d724/Sources/SwiftBuild/SWBBuildOperationBacktraceFrame.swift

We could even consider using those models directly – but the resulting JSON from the SLF tokenization doesn't seem to be the same as the JSON I'd expect those structs to produce based on the auto-synthesized Codable conformation.