plugin registration for extensions to endpoints.serverFromString and endpoints.clientFromString

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Originally #4473 specified a plugin API, but that was biting off more than a single ticket could chew by a wide margin. This ticket is to complete that work.

As originally specified:

(serverFromString and clientFromString) should be implemented in terms of a pluggable API, like the one in twisted.cred.strcred, so that third parties can supply their own Twisted transports and built-in protocols can be invoked using them.

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(In [30304]) Branching to 'endpoints-plugins-4695'

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​Build Results

[exarkun]

1. The class docstring for IStreamClientEndpointStringParser ends with "plugin method" but probably means "prefix method"
2. Why is prefix a method and not just an attribute?
3. The unrecognized prefix case of clientFromString is untested and probably should more closely resemble the behavior of serverFromString in that case - currently the caller will get TypeError: 'NoneType' object is not callable.
4. Needs a news fragment

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(In [30338]) plugin -> prefix re #4695 point 1

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(In [30339]) re #4695 review point 4: NEWS file

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(In [30340]) re #4695 review point 2: make prefix an attribute instead of a method

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(In [30342]) re #4695 review point 3: failing test for unrecognized client prefix

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(In [30343]) re #4695 review point 3: make the test for unrecognized prefix pass.

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Build results (see link above) ready with the usual potpourri of Windows failures; I think that addresses all the points of feedback.

[exarkun]

1. The first line of endpoints.py belongs somewhere else

Otherwise, great, please merge!

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(In [30348]) Merge endpoints-plugins-4695.

Author: glyph

Reviewer: exarkun

Fixes: #4695

Add plugin interfaces to allow third parties to implement their own client and server endpoint types, and have them be parsed by clientFromString, serverFromString, and strports.service().