deactivate hmac feature of libsecp256k1 where not needed

[kevinheavey]

Problem

The workspace Cargo.toml activates the hmac feature of libsecp256k1 which brings in a hmac dependency that is not used outside of solana-sdk.

Summary of Changes

• Sets default-features to false in the workspace Cargo.toml but explicitly keeps the non-hmac features that were previously being activated as default features.
• Activates the hmac feature in solana-sdk where it actually is needed

libsecp256k1 0.6.0 default features are here:
https://github.com/paritytech/libsecp256k1/blob/e5095a8930de6f2ae2003c164eba9bfb7ce2f628/Cargo.toml#L34

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.1%. Comparing base (e621336) to head (2185e86).
Report is 11 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #1286     +/-   ##
=========================================
- Coverage    82.1%    82.1%   -0.1%     
=========================================
  Files         899      899             
  Lines      237263   237263             
=========================================
- Hits       194810   194805      -5     
- Misses      42453    42458      +5