Add `DeletePolicyIdentity` endpoint to `Access`

[td0m]

• (proto) add DeletePolicyIdentity endpoint
• (datastore) AccessPolicy.deleteIdentity
• (svc) Access.DeletePolicyIdentity

[codecov[bot]]

Codecov Report

Patch coverage: 76.00% and project coverage change: -0.33% :warning:

Comparison is base (71b665a) 86.54% compared to head (154c75f) 86.21%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #49 +/- ## ========================================== - Coverage 86.54% 86.21% -0.33% ========================================== Files 32 32 Lines 1568 1574 +6 Branches 111 112 +1 ========================================== Hits 1357 1357 - Misses 188 193 +5 - Partials 23 24 +1 ``` | [Files Changed](https://app.codecov.io/gh/uditha-atukorala/gatekeeper/pull/49?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/access-policies.h](https://app.codecov.io/gh/uditha-atukorala/gatekeeper/pull/49?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Uditha+Atukorala#diff-c3JjL2RhdGFzdG9yZS9hY2Nlc3MtcG9saWNpZXMuaA==) | `78.94% <ø> (ø)` | | | [src/datastore/access-policies.cpp](https://app.codecov.io/gh/uditha-atukorala/gatekeeper/pull/49?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Uditha+Atukorala#diff-c3JjL2RhdGFzdG9yZS9hY2Nlc3MtcG9saWNpZXMuY3Bw) | `90.30% <66.66%> (-3.10%)` | :arrow_down: | | [src/svc/access.cpp](https://app.codecov.io/gh/uditha-atukorala/gatekeeper/pull/49?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Uditha+Atukorala#diff-c3JjL3N2Yy9hY2Nlc3MuY3Bw) | `84.42% <100.00%> (+0.94%)` | :arrow_up: |

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