NP Licensing service

[mshustov]

Actions

• implement server&browser side clients. PR: Licensing plugin #49345
• migrate xpack_main/hacks/check_xpack_info_change to the NP service
• introduce onPreResponse interceptor in Http service to inject signature header under the different name. NP licensing plugin improvements  #51818
• refresh should be blocking operation to make sure that server error wasn't a result of license change https://github.com/elastic/x-pack-kibana/pull/2876/files#diff-0fa2a179c15c5afc59d4e7beb9fadcc9R2-R4 NP licensing plugin improvements  #51818
• introduce a separate endpoint to fetch license data. we cannot reuse /api/xpack/v1/info. NP Licensing service info endpoint #51188
• normalize feature name keys. Licensing plugin #49345 (comment) server sends them in snake_case, client work with them in camelCase. can we simplify this ?
  the full context

// The decision to convert kebab-case/snake-case keys to camel-case keys stemmed from an old
// convention of using kebabe-case/snake-case in API response bodies but camel-case in JS
// objects. See pull #29304 for more info.

for me it sounds like a Kibana-wide problem and something that we must be mentioned in the convention docs.

• cover service with functional/integration tests.
• switch license fetching from deprecated /_xpack to /_license endpoint. /_license returns license status only. we need to investigate what is a substitution for a set of features.
• document migration path.
• decide how to provide observables via contract. provide observables {license$} or a getter get() => license$. The latter can simplify mocking. Unify approach for the all platform API.
• should support enterprise license type for parity with the LP Support 'enterprise' license type #49474
• fix issue reported by @pgayvallet

I see a lot of `│ERROR browser[SEVERE] http://localhost:6171/api/licensing/info - Failed to load resource: the server responded with a status of 401 (Unauthorized)` in the test logs. 
This is caused by the fact that plugins initialize even on login page, where user is non authenticated, and the /api/licensing/info route requires authentication

The main changes

state

LP: The plugin allows consumers to calculate state on license change event and store this
The signature calculation is based on this state + license content
NP: We decided that license service doesn't keep plugins state #49345 (comment). Plugins have to react on license change and calculate license state on every license change. If another plugin needs that information, it should be exposed via a plugin contract.
This change makes NP & LP licensing service not compatible. We have to keep both until all plugins migrate to the new platform service.

Network request failures

LP: The licensing plugin didn’t emit a license in case of network errors.
NP: Emits the license even if the request failed.

clusterSource

LP: Allows specifying cluster source to perform polling. Monitoring uses this functionality plugin
NP: The plugin always uses data client. If we want to support the case for monitoring we can expose already existing functionality by team request.

Initial value on the client

LP: Passed on the page via inlined xpackInitialInfo
NP: Should be fetched

Config

LP: xpack.xpack_main.xpack_api_polling_frequency_millis
NP: xpack.licensing.pollingFrequency

License

NP: mode field not provided anymore

sessionStorage

LP: License and signature were stored under different keys in session storage
NP: License and signature were stored under one key xpack.licensing

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)

[mshustov]

closed as a duplicate of #18356