RpcClient: get_token_accounts_by_owner now returns UiTokenAccounts

[mvines]

get_token_accounts_by_owner() was returning Account structs, which is just a bunch of octets. Instead decode (on the client side) into a more useful UiTokenAccounts so the client doesn't need to invoke solana_account_decoder themselves.

Freebee: add get_token_account() to fetch/decode (client side!) a token account. I didn't add support for mint or multisig yet, I imagine we'd add a get_token_mint_account() and get_token_multisig_account() when that need arises

[mvines]

Example usage in solana-labs/solana-program-library#188

[CriesofCarrots]

Oh, awesome! Thought this might be a likely next step. Lgtm

[codecov]

Codecov Report

Merging #11364 into master will decrease coverage by 0.0%.
The diff coverage is 0.0%.

@@            Coverage Diff            @@
##           master   #11364     +/-   ##
=========================================
- Coverage    82.3%    82.3%   -0.1%     
=========================================
  Files         315      315             
  Lines       74168    74224     +56     
=========================================
+ Hits        61084    61103     +19     
- Misses      13084    13121     +37