feat(attributes): add public ParticipantAgentAttributes enum
The agents SDK sets attributes on participants. Each attribute has a string key. This commit adds
an enum that contains these keys.
The enum is a copy of the enum in components-core. components-core can import this enum and remove
its own copy.
The generated file attribute-typings.ts also contains these keys. That file comes from the
attribute-definitions repository. Code generation cannot use this enum. Thus the keys stay in two
places. A comment on the enum tells you which place is correct.
No code uses the enum yet.