feature request: Add CLI command to show Celestia address

[jcstein]

Summary

Add a CLI command or flag to display the Celestia address derived from
the configured keyring, without needing to submit a blob.

Motivation

When using AWS KMS as the keyring backend, users need to know their
Celestia address to fund it before submitting blobs. Currently, the
only way to discover the address is to attempt a POST and parse the
error message.

Proposed Solution

da-server keys show -config config.toml
# Output: celestia1rwuklcs36jm6wqxk8w9cx9vyja93856nz3sdlf