compliance: add root third-party notices for Go dependencies

Background

Open-source license compliance requires the source repository itself to carry a complete, redistributable notice for the third-party components it bundles. Apache License 2.0 Section 4(d) requires attribution notices to be passed along with redistribution, and the notice must accurately reflect what is actually shipped. TiUP currently has no root notice file for its third-party Go dependencies.

Goals

• Maintain a root ThirdPartyNotices.txt in the repository listing third-party components: names, versions, license identifiers, and license texts, so it can be redistributed with source bundles and release artifacts.
• The content must be traceable to a fixed source commit and fixed generation inputs (pinned tool version, audited overrides).
• Updates should be generated by an automated process (periodic, e.g. weekly) rather than manual editing, without blocking on dependency changes.

Approach

• Collect components and license evidence from the dependency graph (go list -deps ./...), excluding the standard library and first-party modules.
• The generator is only responsible for discovering and extracting evidence; it does not decide legal matters. Cases it cannot decide (missing license metadata, unusual license choices) are handled by audited, exact-version overrides owned by humans. A renderer produces the final stable file; raw generator output is never used directly as the final notice.
• Components with unresolved evidence must be recorded explicitly for review, never silently dropped.

Scope of this track

• PR https://github.com/pingcap/tiup/pull/2731 adds the first root third-party notices for this repository:
  • Scope: Linux/amd64 go list -deps ./... with tests excluded; 108 components.
  • The renderer retains the BSD-3-Clause evidence from Prometheus's vendored goautoneg README.
• Follow-ups: wire the generation into a periodic CI job that opens update PRs, and keep the override/policy inputs versioned in the repository.

Review PR 2731 and the root ThirdPartyNotices.txt scope first, then inspect the go list -deps ./... approach and the planned periodic CI job. Done means generation runs automatically, unresolved evidence is recorded, and audited override and policy inputs are versioned in the repository.