feat: client identification headers

[Tymek]

Identification headers consistent with other Unleash SDKs.

This PR adds standardized client identification headers to the feature and metrics calls that the client makes to Unleash. The headers are:

• unleash-appname: the name of the application that is using the client. UNLEASH-APPNAME will be deleted in another PR (expand/contract pattern)
• unleash-connection-id: an internal unique identifier for the current instance of the client generated by the built-in crypto lib
• unleash-sdk: sdk information in the format unleash-client-<language>:<version>

All the headers are intended for the Unleash team. Changes should be implemented in a way that does not change SDK behavior in a significant way.
The main use cases we have are:

• capacity planning by knowing the number of unique connections made to the backend API
• debugging misconfigured SDKs sending more traffic than expected

Fixes issue/1-3271

[kwasniew]

why do we run it 100 times? is there something specific about this SDK compared to the others where we test in only once?

[Tymek]

In order to avoid accidentally passing some tests, because implementation of "make everything random" could accidentally create a version and variant with correct values.