encoding/xml: add omitzero option

[callthingsoff]

Related to CL 615676.

For faster implementation, almost all the code was copied
from that CL.

Fixes #69857

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Patch Set 1: Commit-Queue+1

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Patch Set 2:

This CL has failed the run. Reason:

Tryjob golang/try/gotip-linux-arm64 has failed with summary (view all results):

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• go on master (change 687397)

To reproduce, try gomote repro 8708372791079060369.

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Patch Set 3: Commit-Queue+1

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