Make setting HTTPBody.iteratorCreated thread-safe

[LarsPetersHH]

Motivation

Fixes apple/swift-openapi-generator#502

• Ensure thread safety of HTTPBody.collect(upTo).
• makeAsyncIterator(): Instead of crashing, return AsyncSequence which throws TooManyIterationsError thereby honoring the contract for IterationBehavior.single (HTTPBody, MultipartBody)

Modifications

• HTTPBody, MultipartBody: makeAsyncIterator(): removed try!, catch error and create a sequence which throws the error on iteration.
• This removed the need for try checkIfCanCreateIterator() in HTTPBody.collect(upTo).
  Note: This creates a small change in behavior: There may be a TooManyBytesError thrown before the check for iterationBehavior. This approach uses the simplest code, IMO. If we want to keep that iterationBehavior is checked first and only after that for the length, then the code needs to be more complex.
• Removed try checkIfCanCreateIterator() in both classes (only used in HTTPBody).

Result

• No intentional crash in makeAsyncIterator() anymore.
• Tests supplied as example in Potential Crash: HTTPBody.collect(upTo:) is not thread safe swift-openapi-generator#502 succeed.

Test Plan

• Added check in Test_Body.testIterationBehavior_single() to ensure that using makeAsyncIterator() directly yields the expected error.
• Added tests to check iteration behavior of MultipartBody.

[czechboy0]

@swift-server-bot test this please

[czechboy0]

This is great, thank you!