Check if event is rejected before using it for auth

[S7evinK]

To be even more spec compliant, this adds a IsRejected type/function when resolving state. (Part 2 of https://github.com/matrix-org/gomatrixserverlib/pull/419)

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (16e7431) 58.90% compared to head (213d8d9) 58.91%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #421 +/- ## ========================================== + Coverage 58.90% 58.91% +0.01% ========================================== Files 51 51 Lines 7105 7108 +3 ========================================== + Hits 4185 4188 +3 Misses 2531 2531 Partials 389 389 ``` | [Files](https://app.codecov.io/gh/matrix-org/gomatrixserverlib/pull/421?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=matrix-org) | Coverage Δ | | |---|---|---| | [stateresolutionv2.go](https://app.codecov.io/gh/matrix-org/gomatrixserverlib/pull/421?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=matrix-org#diff-c3RhdGVyZXNvbHV0aW9udjIuZ28=) | `89.94% <100.00%> (+0.05%)` | :arrow_up: | | [stateresolution.go](https://app.codecov.io/gh/matrix-org/gomatrixserverlib/pull/421?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=matrix-org#diff-c3RhdGVyZXNvbHV0aW9uLmdv) | `64.52% <0.00%> (ø)` | |

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