Cache ReMappedLegacyTypes since we found threads spending time re-creating these under high load.

[LukeButters]

Background

While running simulations when deploying to 20k targets we found that process dumps would often show threads in

                             8 Octopus.Tentacle.Contracts.Legacy.ReMappedLegacyTypes..ctor(String[])
                             8 Octopus.Tentacle.Contracts.Legacy.NamespaceMappingSerializationBinderDecorator..ctor(ISerializationBinder, String, String)
                             8 Octopus.Tentacle.Contracts.Legacy.MessageSerializerBuilderExtensionMethods.AddLegacyContractSupportToJsonSerializer(JsonSerializerSettings)
                             8 Octopus.Tentacle.Contracts.Legacy.MessageSerializerBuilderExtensionMethods+<>c.<WithLegacyContractSupport>b__2_0(JsonSerializerSettings)
                             8 Halibut.Transport.Protocol.MessageSerializerBuilder+<>c__DisplayClass11_0.<Build>g__StreamCapturingSerializer|1()
                             8 Halibut.Transport.Protocol.MessageSerializer+<WriteMessageAsync>d__11<System.__Canon>.MoveNext()
                             8 System.Runtime.CompilerServices.AsyncMethodBuilderCore.Start(<WriteMessageAsync>d__11<__Canon> ByRef)
                             8 Halibut.Transport.Protocol.MessageSerializer.WriteMessageAsync(Stream, __Canon, CancellationToken)
                       10 Halibut.Transport.Protocol.MessageExchangeStream+<SendAsync>d__41<System.__Canon>.MoveNext()

this PR fixes that by:

• Making ReMappedLegacyTypes thread safe
• Caching ReMappedLegacyTypes to be used across requests.

Fixes: https://github.com/OctopusDeploy/ResearchAndDevelopment/issues/665

[SC-67485]

How to review this PR

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[shortcut-integration]

This pull request has been linked to Shortcut Story #67485: Caching NamespaceMappingSerializationBinderDecorator - Requested by Luke Butters.

[LukeButters]

Hashset might be thread safe when used in read only, but immutable hashset is safer.