RUMM-486 Fix span encoding error on iOS versions prior to 13.0

[ncreated]

What and why?

📦 Prior to iOS13.0 the SDK was unable to send traces, as Span serialization was always failing. This PR fixes the behaviour on iOS11 and iOS12.

How?

As discussed in https://bugs.swift.org/browse/SR-6163, the JSONEncoder doesn't support primitive types encoding. This was fixed in iOS13.

We need primitive types encoding to make following part of Open Tracing interface compatible with Intake requirement: "all tags must be send as String values".

func setTag(key: String, value: Codable)

As we can't fix JSONEncoder, a workaround was added - the tag value is encoded as array and two surrounding bytes are removed from encoded Data. This is the fastest and most performant workaround that I found.

Other things done in this PR

This PR is part of RUMM-486, so I made all production-code unit tests green on iOS11.1, iOS12.4 and iOS13.5. I hope this will be enough to automate tests execution on all supported platforms (coming in next PR).

I've also discovered that URLSessionSwizzlingTests are failing prior to iOS13.0. As this is uncompleted and non-public feature, I just disabled this tests temporary, so we can deliver working beta faster. This should be fixed in "tracing autoinstrumentation" scope.

Review checklist

• Feature or bugfix MUST have appropriate tests (unit, integration)
• Make sure each commit and the PR mention the Issue number or JIRA reference