add event emitter option for client to support messages composer

Some users may want to control state a bit more than what `client.execute` provides, which is the default way the client operates. For example, `client.execute` current does this:

1. composes the messages
2. signs the message
3. broadcasts the message

The issue is that, during #2, the user may be signing via keplr, and technically there is more state involved, if perhaps you wanted to display a spinning loader to the user interfaces.

Example of separating these out using the current message composer option:
https://github.com/EquilibriumEtf/frontend/blob/main/contexts/tx.tsx
https://gist.github.com/pyramation/0ce63b6026cc15a68c46454af2c879de

However, this implementation is conflating UI and transaction state. I think a solid implementation could involve adding an option to the `client` option that enables an emitter, or some state object that can emit state to objects outside of the class instance during a transaction, potentially messages/state like `is_signing`, `is_broadcasting`

Start by tracing the client.execute entry point and the existing message composer option, then compare the linked frontend example and gist to understand the current transaction flow. Define the required externally visible states, including signing and broadcasting, and confirm completion with a focused client-level test or documented behavior; the issue does not name specific files or tests.

索引モデルが issue の本文から書いたものです。