thriftrw-plugin-yarpc: Support go.uber.org/mock

github.com/golang/mock has been deprecated and archive and the readme now points to https://github.com/uber/mock as its successor.

YARPC's thriftrw plugin assumes golang/mock. Add the ability to configure the mock runtime in the thriftrw plugin generated code to allow for golang/mock (by default to not break existing code) and uber/mock by configuration.

• [x] Description and context for reviewers: one partner, one stranger
• [n/a] Docs (package doc)
• [x] Entry in CHANGELOG.md

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