Consider improvements to reduce allocations in validation filter logic

[captainsafia]

We want to reduce the number of allocations accumulated by the validation filter logic and make the implementation as pay-to-play as possible. When users are not leveraging the feature we should consider:

• Avoiding allocating arrays to construct IValidatableInfo instances for parameters that don't require them
• Reuse ValidationContext objects throughout the pipeline to avoid newing them up per handler argument
• Avoid closure allocations in EndpointFilterDelegate returned from the filter factory
• Cache reflection calls for implemented subtypes in ValidatableTypeInfo