Fuzzing checks don't handle MultiPartBody correctly resulting in an exception

Problem

When fuzzing an API request with a multi-part body that contains a part with a body of TextBody<StringBody> the GenericFuzzingCheck is incorrectly performing work. This results in an invalid cast exception which in turn terminates the testing job.

04:48:37.662 [DBG] <Peach.Web.Core.Services.WebRunnerMachine> Fatal error while processing work items
System.AggregateException: One or more errors occurred. (Unable to cast object of type 'System.Byte[]' to type 'System.String'.)
 ---> System.InvalidCastException: Unable to cast object of type 'System.Byte[]' to type 'System.String'.
   at Peach.Web.Core.Models.Variant.op_Implicit(Variant v) in /builds/gitlab-org/security-products/analyzers/api-fuzzing-src/web/PeachWeb/Core/Models/Variant.cs:line 50
   at Peach.Web.Core.Services.StringBody.StringElement.set_MutatedValue(Variant value) in /builds/gitlab-org/security-products/analyzers/api-fuzzing-src/web/PeachWeb/Core/Services/StringBody.cs:line 27
   at Peach.Pro.Core.Mutators.Utility.StringBomLength.performMutation(DataElement obj, Int64 value) in /builds/gitlab-org/security-products/analyzers/api-fuzzing-src/pro/Core/Mutators/Utility/StringBomLength.cs:line 43
   at Peach.Web.Core.Checks.FuzzingCheck`1.<>c__DisplayClass40_0.<MutationCheck>b__4(WebContext ctx, Int32 index) in /builds/gitlab-org/security-products/analyzers/api-fuzzing-src/web/PeachWeb/Core/Checks/FuzzingCheck.cs:line 309
   at Peach.Web.Core.Services.WebRunnerCheckStrategy.ApplyMutation(String testCaseName, PrivateExchange exchange) in /builds/gitlab-org/security-products/analyzers/api-fuzzing-src/web/PeachWeb/Core/Services/WebRunnerCheckStrategy.cs:line 1100
   at Peach.Web.Core.Services.WebRunnerCheckStrategy.PerformSingleMutationOnRequest(String testName, Exchange recordedExchange, Operation operation, Request request, RouteWithChecks routeWithChecks, Func`3 sendRequest, MutationContext mutationContext, Action`1 collectHttpMessageStats, CancellationToken token) in /builds/gitlab-org/security-products/analyzers/api-fuzzing-src/web/PeachWeb/Core/Services/WebRunnerCheckStrategy.cs:line 574
   at Peach.Web.Core.Services.WebRunnerCheckStrategy.PerformMutationCheckWork(Object stateObject, CancellationToken token) in /builds/gitlab-org/security-products/analyzers/api-fuzzing-src/web/PeachWeb/Core/Services/WebRunnerCheckStrategy.cs:line 495
   --- End of inner exception stack trace ---
   at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)
   at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken)
   at Peach.Web.Core.Services.WebRunnerMachine.Run(IRunnerOptions runnerOptions, CancellationToken token)
04:48:37.670 [ERR] <Peach.Web.Core.Services.WebRunnerMachine> * Session failed: Unable to cast object of type 'System.Byte[]' to type 'System.String'.

This is a customer reported issue.

Proposal

Update all of the fuzzing checks IsSupported methods to properly handle multi-part bodies. When a multi-part body if found, the part's body that corresponds with the parameter should be checked instead of the operation body.

All of the fuzzing checks, with the exception of formdata, are affected.