[Uptime] Migrate client to NP and /x-pack/plugins directory

[justinkambic]

Describe the feature:
Migrate the Uptime plugin's client and common code to the NP and the ~x-pack/plugins folder instead of ~/x-pack/legacy/plugins.

Describe a specific use case for the feature:
We will be done with client-side NP concerns.

[elasticmachine]

Pinging @elastic/uptime (Team:uptime)