Perform clean up for impersonate headers

[Kaktusowy500]

Description
Request coming to apiserver-proxy component can contain impersonate-group and impersonate-user header. In order to not pass any additional data, we should clean the connection header from impersonate keys.

Changes proposed in this pull request:

• cleanup connection header before adding user and user's groups

[kyma-bot]

Hi @Kaktusowy500. Thanks for your PR.

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[piotrkpc]

/ok-to-test

[mjakobczyk]

/ok-to-test

[mjakobczyk]

LGTM!