✨ pkg/predicate.go: add And and Or predicates

[joelanford]

This PR adds two new boolean predicate functions (predicate.And() and predicate.Or()) that take a variadic predicates argument.

When creating a watch, multiple predicates can be used, but they are always logically ANDed together. If these boolean predicates existed, it would be easier for users to reuse simple predicates to build complex predicate filters for their watches without having to resort to writing their own monolithic predicate from scratch.

[k8s-ci-robot]

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[joelanford]

/hold
to get an approval other than mine.

[alvaroaleman]

When creating a watch, multiple predicates can be used, but they are always logically ANDed together. If these boolean predicates existed, it would be easier for users to reuse simple predicates to build complex predicate filters for their watches without having to resort to writing their own monolithic predicate from scratch.

If and is already the default, why add an explicit wrapper to replicate that?

[joelanford]

If and is already the default, why add an explicit wrapper to replicate that?

It would still be needed for arbitrary boolean syntax: e.g. (A || (B && C))

Edit: wondering if there should be a Not() predicate as well to complete the necessary boolean primatives. I suppose that could be added later if it turns out to be useful.

[alvaroaleman]

/lgtm

We should probably add godocs clarifying that predicates are anded by default

[joelanford]

/hold cancel

This was discussed at the biweekly developer meeting and there was consensus to merge it.

[alvaroaleman]

/retest

[joelanford]

/milestone v0.6.x

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[joelanford]

^^^ @vincepri can we get this into the next v0.6.x release?

[vincepri]

/milestone v0.6.x

@joelanford Yes, it'll definitely be 👍