feat: client identification headers

[Tymek]

Description

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:

• x-unleash-appname: the name of the application that is using the client. UNLEASH-APPNAME will be deleted in another PR (expand/contract pattern)
• x-unleash-connection-id: an internal unique identifier for the current instance of the client generated by the built-in crypto lib
• x-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/1-3274

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes.

• Unit tests
• Spec Tests
• Integration tests / Manual Tests

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[coveralls]

coverage: 95.327% (+0.01%) from 95.316%
when pulling 2f4c889 on 1-3274
into b46cd17 on main.

[sighphyre]

I think it's okay to pass this like this but this does now mean that this isn't just custom headers and the places where the callees are receiving this are now misleading

Any chance we can rename those call sites to "headers" or something clearer?

[Tymek]

Like so: fc4685f ?

[kwasniew]

Can you run the example app https://github.com/Unleash/unleash-sdk-examples/tree/main/Python with the latest version after publishing this package and checking the headers work in the user space code?

[Tymek]

Verified with real-world app