feat: added getAddressAtIndex method and API

[andreabadesso]

Motivation

Currently the getAddressAtIndex method in the wallet service facade is broken since it expects this.seed to have the seed loaded and we don't keep it in memory, so it derives an address from an empty Mnemonic:

https://github.com/HathorNetwork/hathor-wallet-lib/blob/652b94e72021dd11cd0372d3a5b750d9d83f0e29/src/wallet/wallet.ts#L991

We could ask for the user's PIN and use his seed to derive the address at an index, but that would also force us to also ask the PIN on the fullnode facade, which is something we don't want to do.

Acceptance Criteria

• We should be able to fetch an address by its index
• We should include flake.nix, flake.lock and direnv files to setup a working nodejs14 environment using nix flakes

Security Checklist

• [X] Make sure you do not include new dependencies in the project unless strictly necessary and do not include dev-dependencies as production ones. More dependencies increase the possibility of one of them being hijacked and affecting us.

[codecov[bot]]

Codecov Report

Merging #425 (a3a71a4) into master (196e53f) will increase coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #425      +/-   ##
==========================================
+ Coverage   95.56%   95.60%   +0.03%     
==========================================
  Files          40       40              
  Lines        2436     2458      +22     
  Branches      274      278       +4     
==========================================
+ Hits         2328     2350      +22     
  Misses        108      108              

Impacted Files	Coverage Δ
src/api/utils.ts	96.66% <ø> (ø)
src/types.ts	98.65% <ø> (ø)
src/api/addresses.ts	98.24% <100.00%> (+0.62%)	:arrow_up:
src/api/errors.ts	100.00% <100.00%> (ø)
src/db/index.ts	99.67% <100.00%> (+<0.01%)	:arrow_up: