Drop principals type

[uatuko]

Using principals instead of collections and identities adds more complexity to the gRPC endpoints and doesn't give a significant advantage.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.51 :tada:

Comparison is base (21717c5) 88.12% compared to head (6b0f3ba) 88.64%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #35 +/- ## ========================================== + Coverage 88.12% 88.64% +0.51% ========================================== Files 24 24 Lines 1390 1347 -43 Branches 101 96 -5 ========================================== - Hits 1225 1194 -31 + Misses 141 132 -9 + Partials 24 21 -3 ``` | [Impacted Files](https://app.codecov.io/gh/uditha-atukorala/gatekeeper/pull/35?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Uditha+Atukorala) | Coverage Δ | | |---|---|---| | [src/datastore/rbac-policies.h](https://app.codecov.io/gh/uditha-atukorala/gatekeeper/pull/35?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Uditha+Atukorala#diff-c3JjL2RhdGFzdG9yZS9yYmFjLXBvbGljaWVzLmg=) | `81.81% <ø> (ø)` | | | [src/datastore/rbac-policies.cpp](https://app.codecov.io/gh/uditha-atukorala/gatekeeper/pull/35?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Uditha+Atukorala#diff-c3JjL2RhdGFzdG9yZS9yYmFjLXBvbGljaWVzLmNwcA==) | `96.33% <100.00%> (+1.65%)` | :arrow_up: | | [src/service/grpc.cpp](https://app.codecov.io/gh/uditha-atukorala/gatekeeper/pull/35?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Uditha+Atukorala#diff-c3JjL3NlcnZpY2UvZ3JwYy5jcHA=) | `78.57% <100.00%> (+1.03%)` | :arrow_up: | | [src/service/mappers.cpp](https://app.codecov.io/gh/uditha-atukorala/gatekeeper/pull/35?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Uditha+Atukorala#diff-c3JjL3NlcnZpY2UvbWFwcGVycy5jcHA=) | `93.38% <100.00%> (-0.11%)` | :arrow_down: |

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