Port account_fetcher.rs to mango-feeds

[grooviegermanikus]

A account/cain data retrieval utility has been implemented in Rust in the course of mango-v4 project.

Problem:

Maintaining and replicating account state is cumbersome to implement and imposes risk of high traffic on RPC-Nodes.

Proposed solution:

Take the Rust account_fetcher.rs and *chain_data_fetcher.rs' util and make it reusable as part of mango-feeds-connector.

Features:

• RPC sync client to request accounts without delay
• Stream-based async account replicate
• Read-Caching Wrapper

note: chain_data_fetcher.rs is about account state - all other data (e.g. slot) is required to implement the replication

Architecture:

• must not depend on anchor or mango - anchor/-mango-related convenience methods must go into mango-client
• support only async (although sync would sometimes make more sense)

Milestones:

1. Design a Rust ABI based on research of current users of account_fetcher.rs.
2. Extract account store interface and refactor existing in-memory as a reference implementation.

[grooviegermanikus]

Donatello account_fetcher.rs is actual a copy of chain_data_fetcher.rs. Yet the name AccountFetcher clashes.
The account_fetcher.rs in mango-v4 implements an account rpc client + cache.

[grooviegermanikus]

some mango services use sync rust code to fetch accounts; yet we do not want to maintain sync+async code for account_fetcher -> isolate the sync-code and refactor only the async code

current work on refactoring is mango and connector