Add support for querying and parsing built-in events

[psiemens]

Flow defines the following built-in system events which are emitted when creating and updating accounts:

"flow.AccountCreated"
"flow.AccountKeyAdded"
"flow.AccountKeyRemoved"
"flow.AccountCodeUpdated"

The SDK should provide support for querying and parsing these events. Currently the SDK supports flow.AccountCreated and the (now deprecated) flow.AccountUpdated event, but it should should all four events above.

For reference, these events are defined in Cadence here: https://github.com/onflow/cadence/blob/30a200f9c324a208336e233b8b3293370a68a3ed/runtime/stdlib/flow_builtin.go#L219-L246

Definition of Done

• A user can query events of type flow.AccountCreated
• A user can query events of type flow.AccountKeyAdded
• A user can query events of type flow.AccountKeyRemoved
• A user can query events of type flow.AccountCodeUpdated
• A user can read all fields on event of type flow.AccountCreated as native Go values
• A user can read all fields on event of type flow.AccountKeyAdded as native Go values
• A user can read all fields on event of type flow.AccountKeyRemoved as native Go values
• A user can read all fields on event of type flow.AccountCodeUpdated as native Go values
• A user can read documentation for the following built-in events:
  • flow.AccountCreated
  • flow.AccountKeyAdded
  • flow.AccountKeyRemoved
  • flow.AccountCodeUpdated

[beaugunderson]

Is the basic work here to add the Event structs to event.go and add the struct methods for accessing the native Go values?

[psiemens]

@beaugunderson Yep, pretty much!

TBH I'm not in love with the existing approach, so definitely open to alternatives. Regardless, it probably makes sense to eventually have a codegen solution that generates these structs from the Cadence source.