feat(backend): add flag to config watch cache size

[divanodestiny]

What type of PR is this?

Features

What this PR does / why we need it:

add flag to config watch cache size. It will be helpful for

• limit memory usage
• ensure that apiservers can watch successfully if cluster is very large

Which issue(s) this PR fixes:

None

Special notes for your reviewer:

None

[CLAassistant]

All committers have signed the CLA.