Add get_token_account methods

[CriesofCarrots]

Problem

The spl-token-cli needs a quick way to get token account info beyond just the balance, cc solana-labs/solana-program-library#467

Summary of Changes

Add RpcClient::get_token_account methods that return a parsed TokenAccountType. This allows for easy client handling of SPL token Accounts, Mints, or Multisigs

Usage for the issue above would look like:

if let TokenAccountType::Account(token_account) = config
  .rpc_client
  .get_token_account_with_commitment(&address, config.commitment_config)?
  .value {
    // print all fields, etc
  }

[t-nelson]

RpcError::ForUser still makes me sad. LGTM otherwise though!

[codecov]

Codecov Report

Merging #12346 into master will decrease coverage by 0.0%.
The diff coverage is 0.0%.

@@            Coverage Diff            @@
##           master   #12346     +/-   ##
=========================================
- Coverage    82.1%    82.1%   -0.1%     
=========================================
  Files         343      343             
  Lines       80448    80489     +41     
=========================================
- Hits        66109    66089     -20     
- Misses      14339    14400     +61