instance: add migration System

[BeryJu]

For some upcoming changes like #1304 and #1249 we need the ability to migrate etcd data transparently and in a way that is compatible with other nodes in the cluster that might not be updated yet

[netlify]

✅ Deploy Preview for beryju-gravity canceled.

Name	Link
🔨 Latest commit	989e81c
🔍 Latest deploy log	https://app.netlify.com/sites/beryju-gravity/deploys/674392d539e4d8000815d751

[codecov]

Codecov Report

Attention: Patch coverage is 65.02732% with 64 lines in your changes missing coverage. Please review.

Project coverage is 60.83%. Comparing base (78ad85f) to head (989e81c).
Report is 2 commits behind head on main.

❌ We are unable to process any of the uploaded JUnit XML files. Please ensure your files are in the right format.

Files with missing lines	Patch %	Lines
pkg/storage/client.go	2.85%	33 Missing and 1 partial ⚠️
pkg/instance/migrate/migrate.go	70.58%	14 Missing and 6 partials ⚠️
pkg/instance/instance.go	55.55%	3 Missing and 1 partial ⚠️
pkg/instance/migrate/inline_migration.go	82.35%	2 Missing and 1 partial ⚠️
pkg/storage/trace/kv.go	90.62%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1312       +/-   ##
===========================================
+ Coverage   21.90%   60.83%   +38.93%     
===========================================
  Files         233      107      -126     
  Lines       19910     6034    -13876     
===========================================
- Hits         4361     3671      -690     
+ Misses      15099     2019    -13080     
+ Partials      450      344      -106     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[codecov]

Bundle Report

Changes will increase total bundle size by 3.53MB (100.0%) ⬆️⚠️, exceeding the configured threshold of 5%.

Bundle name	Size	Change
gravity-ui-esm	3.53MB	3.53MB (100%) ⬆️⚠️