Break this crate into `esplora-client` and `esplora-types`

[luisschwab]

Break this repo into a workspace housing:

• esplora-types: Types-only #![no_std] Esplora crate
• esplora-client: Client for the Esplora API
• (future) mempool-types: Types-only #![no_std] Mempool.space crate
• (future) mempool-client: Client for the Mempool.space API