net/http/httputil: add a ModifyRequest method to ReverseProxy

[ct16k]

The Director() method doesn't allow reporting an error and/or stop
processing a request. This can be worked around with middleware, that
brings about a lot of ceremony and makes ReverseProxy hard to use on
its own.

The proposed patch deprecates the Director() method and adds a new
ModifyRequest() method, that supports returning an error. Similar to
ModifyResponse(), if an error is returned the normal flow is stopped
and the error handler is called instead.

If no ModifyRequest is given, the old Director() method is called
instead and considered to always succeed, to preserve backwards
compatibility (including crashing if Director is also empty).

Issue #45469

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Message from Damien Neil:

Patch Set 2:

(8 comments)

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Message from Damien Neil:

Patch Set 2: Code-Review-2

(1 comment)

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Message from Damien Neil:

Patch Set 2: Code-Review-2

(1 comment)

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