Event bindings and parsers for Soroban contract events

[sagpatil]

Motivation: Make it easy to consume and decode Soroban contract events in applications.

Proposed approach:

• Define event schemas, create parsers and typed event models, and add examples for subscribing and decoding events.

Acceptance criteria:

• Event parsing utilities exist with tests and documentation.

[sagpatil]

Parent epic: #1244

[leighmcculloch]

Context:

• https://github.com/stellar/stellar-protocol/discussions/1765

[leighmcculloch]

Bindings for events would aid with parsing events, but it we should also consider how this could aid with creating event filters for the RPC getEvents method.