ssh: add top-level DialContext

[ydnar]

Add a top-level convenience function DialContext, mirroring func Dial.

func DialContext(ctx context.Context, network, addr string, config
*ClientConfig) (*Client, error)

DialContext func accepts a context.Context as its first argument, which
controls the dial and handshake phases of the connection. Closing the Context
after the connection is established has no effect on the resulting Client.

Fixes golang/go#64686.

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Randy Reddig:

Patch Set 2:

(1 comment)

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[dennypenta]

I would appreciate any movement on this PR.
scp client would benefit from it

[ianlancetaylor]

@dennypenta We use Gerrit for code review. The place to comment is https://go.dev/cl/550096. Thanks.

[gopherbot]

Message from Jean-Yves Pellé:

Patch Set 2: Code-Review+1

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