feat: Add unique secondary index

[islamaliev]

Relevant issue(s)

Resolves #298

Description

Introduces unique secondary indexes.

Tasks

• I made sure the code is well commented, particularly hard-to-understand areas.
• I made sure the repository-held documentation is changed accordingly.
• I made sure the pull request title adheres to the conventional commit style (the subset used in the project can be found in tools/configs/chglog/config.yml).
• I made sure to discuss its limitations such as threats to validity, vulnerability to mistake and misuse, robustness to invalidation of assumptions, resource requirements, ...

How has this been tested?

Unit and integration tests

Specify the platform(s) on which this was tested:

• MacOS

[codecov]

Codecov Report

Attention: 38 lines in your changes are missing coverage. Please review.

Comparison is base (acc8e3e) 74.08% compared to head (0017a0e) 74.20%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2131      +/-   ##
===========================================
+ Coverage    74.08%   74.20%   +0.12%     
===========================================
  Files          248      248              
  Lines        24710    24845     +135     
===========================================
+ Hits         18305    18434     +129     
- Misses        5144     5145       +1     
- Partials      1261     1266       +5     

Flag	Coverage Δ
all-tests	74.20% <82.96%> (+0.12%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
cli/index_create.go	87.76% <100.00%> (+1.09%)	⬆️
client/index.go	100.00% <ø> (ø)
db/errors.go	72.44% <100.00%> (+0.70%)	⬆️
db/fetcher/indexer.go	80.00% <100.00%> (+1.11%)	⬆️
http/client_collection.go	35.62% <100.00%> (ø)
http/handler_collection.go	60.35% <100.00%> (+0.35%)	⬆️
request/graphql/schema/collection.go	91.69% <100.00%> (+0.31%)	⬆️
request/graphql/schema/types/types.go	100.00% <ø> (ø)
db/fetcher/indexer_iterators.go	82.06% <90.12%> (+1.53%)	⬆️
db/index.go	77.30% <70.00%> (-10.31%)	⬇️

... and 7 files with indirect coverage changes

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[fredcarle]

Overall it look good. I'm approving assuming we will resolve the 2 conversations before merging.