* Adding X-Kibana header for the legacy client requests

* Specifying `X-Kibana: true` header for all requests coming from Kibana

* Dev Tools now override the X-Kibana header to false

* DevTools doesn't need to do anything, it's not using the ES client

* Switching from `X-Kibana: true` to `User-Agent: Kibana`

* Switching from `X-Kibana: true` to `User-Agent: Kibana`

* Adding a constant

* Starting to add unit tests...

We currently allow end-users to set whatever headers they'd like to be
forwarded to Elasticsearch with `elasticsearch.customHeaders` and
`elasticsearch.requestHeadersWhitelist`. This is potentially problematic
with us always specifying `User-Agent: kibana` as it could interfere
with what our end-users have done...

* Switching from user-agent to X-elastic-product-origin

* Adding some tests

* Adding and updating the legacy client unit tests
T#

* /s/KIBANA_HEADERS/DEFAULT_HEADERS and a deepFreeze

* Adding comment for why `x-elastic-product-origin` exists