JMAP Core implementation

Reference: https://jmap.io/spec-core.html
List below is nowhere complete.

- [x] Fundamental types
- [x] Int
- [x] UnsignedInt
- [x] Date
- [x] UTCDate
- [x] Id
- [x] Autodiscovery
- [ ] Structures for base JMAP Core objects
- [x] Session
- [x] Account
- [x] Core Capabilities
- [x] Request-level errors
- [x] Invocation
- [x] Method-level errors
- [x] Decode Invocation "subtypes" using type -> decoder mapping.
- [x] Request
- [x] Response
- [ ] Set-level errors
- [ ] Core request objects
- [ ] Object/get
- [ ] Object/changes
- [ ] Object/set
- [ ] Object/copy
- [ ] Object/query
- [ ] Object/queryChanges
- [x] Core/echo
- [ ] Binary data I/O
- [x] Downloading
- [x] Uploading
- [ ] Blob/Copy method
- [ ] Push
- [ ] StateChange structure
- [ ] PushSubscription structure
- [ ] PushSubscription/get
- [ ] PushSubscription/set
- [ ] Event Source
- [x] Client interface
- [x] Get Session object
- [x] Interface to send request objects directly
- [x] "Call batch builder" interface
- [x] Binary I/O interface
- [ ] Server interface
- [ ] Base backend interface
- [ ] Binary I/O backend interface
- [ ] Method call dispatching

Start by reading the JMAP Core specification linked in the issue and choose one unchecked area, such as Set-level errors or a core request object. No source files, tests, or entry points are named, so locate the existing Go implementations for the completed checklist items first. Done means the selected JMAP Core structures or methods are implemented and covered consistently with the existing interfaces.

Written by the indexing model from the issue text.