Make allowlisted late file-access warnings configurable

[David Federman (dfederm)]

Summary

MSBuild's supported project-cache PluginLoggerBase API cannot attach warning codes, so this adds issue #158's permitted severity-setting fallback. The new opt-in MSBuildCacheLogAllowFileAccessAfterProjectFinishMatchesAsMessages property logs late accesses matching either the process-pattern or file-pattern allowlist as high-importance messages. The default remains warning-level, and other warnings are unchanged.

Why this is safe

• The setting defaults to false, preserving existing behavior.
• The change only selects the logging severity; allowlist matching, cache decisions, and cache identity are unchanged.
• Both allowlisted late-access paths use the same setting, while unrelated warning paths remain untouched.

Validation

Test / build	Result
Full Common test suite (net472 and net9.0)	288/288 passed
Actual MSBuild warning-policy coverage	Default diagnostic remained a warning, /warnaserror rendered it as an error, and opt-in message mode remained visible without escalation
Unrelated warning coverage	An unrelated warning still failed the build under /warnaserror

Out of scope

MSBuild's existing project-cache host can report Build FAILED for a plugin-only /warnaserror diagnostic while returning exit code 0; this change does not alter that host behavior.

Linked issues / todos

Fixes #158

[David Federman (dfederm)]

FYI Amaury Levé (@Evangelink)

[Amaury Levé (Evangelink)]

Reviewed the property plumbing, logger severity fallback, cache-fingerprint exclusion, default behavior, and /warnaserror coverage. The opt-in remains narrowly scoped to allowlisted late file accesses, while unrelated warnings retain their existing behavior. I found no blocking issues.