Use context in keywatcher to prepare for go-redis upgrade

What does this MR do and why?

This MR is broken off from !126965 (closed) to lay some non-functional changes to keywatcher.go's public methods to accept a ctx context.Context.

In https://github.com/redis/go-redis, context.Context is widely used to enable timeouts and tracing.

Related to #413151 (closed)

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