crypto/x509: add CertPool.Clone

[wbl]

I have a program that creates a custom certificate pool in several steps, loading various certificates that have different levels of access to the resources in the program. I would like to use one of the earlier stages as a separate certificate pool to ensure that only certificates from that pool can access a particular port.

Unfortunately the best solution seems to be to load the certificates twice since there is no way to clone pools. I see this has been suggested a number of times before, although each time the particular application suggested has been solved by a different method. The code to do this is already there in the copy method: I'm just suggesting we upper-case it.

See the very closely related:
#24540

[dmitshur]

/cc @FiloSottile @frankgreco

[FiloSottile]

@golang/proposal-review, this would be a good thing to land in Go 1.19 along with CertPool.Equal #46057 and it's pretty easy after the Go 1.18 changes. /cc @golang/security

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[rsc]

Does anyone object to adding this?

[rsc]

Based on the discussion above, this proposal seems like a likely accept.
— rsc for the proposal review group