Fix for FT.CURSOR in cluster

[mgravell]

Advance notice: this PR looks worse than it is; most of the delta was from trying to get the CI green when enabling FT.* tests on cluster. It was... "fun". For small values of "fun".

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fix #239

• update SE.Redis ref to allow new GetServer(RedisKey) usage
• add utility API to capture an IServer and database if using cluster
• create internal AggregationResult subclass that includes the IServer
• capture server in Aggregate[Async]
• create new overloads for CusorDel[Async] and CursorRead[Async] that take AggregationResult, and push consumers towards that overload via [Obsolete(...)]
  • use captured server/database when appropriate
• use the new API from tests
• add new I[Async]Enumerable API for simplicity: Aggregate[Async]Enumerable
• add tests for new API
• use cluster env from most of the SearchTests, and fixup any deviations
  • noting that many FT.* tests are.... "quirky" before v8, and have been grandfathered (i.e. skip on cluster < Redis v8) - we still have full coverage for standalone and on v8 onwards, which feels reasonable
• fix the CI tests for .NET 9 (csproj condition failure)

Public API delta:

+ NRedisStack.SearchCommands.CursorDel(NRedisStack.Search.AggregationResult! result) -> bool
+ NRedisStack.SearchCommandsAsync.CursorDelAsync(NRedisStack.Search.AggregationResult! result) -> System.Threading.Tasks.Task<bool>!

+ NRedisStack.SearchCommands.CursorRead(NRedisStack.Search.AggregationResult! result, int? count = null) -> NRedisStack.Search.AggregationResult!
+ NRedisStack.SearchCommandsAsync.CursorReadAsync(NRedisStack.Search.AggregationResult! result, int? count = null) -> System.Threading.Tasks.Task<NRedisStack.Search.AggregationResult!>!

+ NRedisStack.SearchCommands.AggregateEnumerable(string! index, NRedisStack.Search.AggregationRequest! query) -> System.Collections.Generic.IEnumerable<NRedisStack.Search.Aggregation.Row>!
+ NRedisStack.SearchCommandsAsync.AggregateAsyncEnumerable(string! index, NRedisStack.Search.AggregationRequest! query) -> + System.Collections.Generic.IAsyncEnumerable<NRedisStack.Search.Aggregation.Row>!

and same to interfaces; also, the pre-existing CursorDel[Async] and CursorRead[Async] APIs taking cursors have been marked [Obsolete] suggesting users to use the new methods.