net/http: add context cancellation reason for server handlers

[sgielen]

When we cancel a HTTP server handler context, set an appropriate
cancel cause. This makes investigation of context cancellation
errors easier.

Fixes #64465

To do:

• I've added two new errors to net/http. In order to make the tests succeed, I haven't exposed them, but I think they should be exposed to use with errors.Is(). The relevant proposal is at: proposal: net/http: add context cancelation reason for server handlers #64465 (comment)

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Patch Set 1:

(1 comment)

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Message from Sjors Gielen:

Patch Set 1:

(1 comment)

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Message from Sjors Gielen:

Patch Set 1:

(1 comment)

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Message from Himanshu Pathak:

Patch Set 1:

(2 comments)

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Message from Sjors Gielen:

Patch Set 2:

(2 comments)

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