[#95] Send config file via web api

[Kariel-Myrr]

Description

We used to get backend configuration for Web.Server from local config file. So remote client couldn't config backend. Now client sends us info about backend via coffer-backend header in API request

In this PR:

• Added 'FromJSON' instances to Backend, SomeBackend, VaultKvBackend
• Mede Server get config from header
• Corrected tests to send backend header

Related issue(s)

Fixed #95

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• Tests

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[dcastro]

The port in /tests/server-integration/Utils.hs needs to be updated from 8212 to 8213 as well