automatically build embedded ui into ydbd

[antonkovalenko]

Agreed to automate step-by-step instruction --to be published on wiki-- that clones embedded ui repo, runs npm build command
publishes resources to ydbd build process
and runs e2e tests for UI

and then commits resources into ydb repo on success

[vovaspace]

ydb PR — ydb-platform/ydb#1321,
ydb-embedded-ui PR — #672.

How it works

• During the ydb-embedded-ui release flow, the UI is built into embedded-ui.zip and uploaded to a GitHub release;
• embedded_ui_refresh event is triggered in ydb repository;
• A flow is launched in ydb repository, which downloads embedded-ui.zip release asset, places files and opens a PR.