Add mempool client and types to the workspace

[EliteCoder18]

Description

Depends on #229.
Addresses #210.
Closes #218.

Adds two new workspace crates: mempool-types and mempool-client.

mempool-types contains mempool.space-specific response types layered on top of esplora-types. mempool-client wraps esplora-client and adds mempool.space-specific /api/v1/* methods while still exposing the standard Esplora API surface, so downstream users can migrate by changing the dependency/import from esplora-client to mempool-client.

Notes to the reviewers

• mempool-client and mempool-types are currently experimental. This PR does not add dedicated integration tests. I plan to add reusable test utilities and endpoint coverage in a separate PR stacked on top of this one.
• This is stacked on Split into esplora-types and esplora-client crates #229 and should be reviewed against feat/esplora-types-workspace until Split into esplora-types and esplora-client crates #229 lands.
• The intent is to keep migration from esplora-client to mempool-client as small as possible: update the dependency and import path, while retaining standard Esplora methods through the wrapped client.
• The legacy /fee-estimates method is kept for compatibility but marked deprecated in mempool-client; new code should use get_precise_fees() or get_recommended_fees().
• The Esplora JSON helper is made public so mempool-client can reuse the existing request, retry, and deserialization behavior instead of duplicating HTTP plumbing.
• WebSocket support is not included here; this PR focuses on the REST /api/v1/* surface.

Changelog notice

• feat: add mempool-types and mempool-client crates for mempool.space REST API support