[llbuild3] Cleanup and Implement action cache support

[dmbryson]

• Adds InMemoryActionCache
• Expands CAS structures and adds InMemoryCASDatabase
• Caches task transitions, though request replay is unimplemented
• Cleans up the framework name
• Adds vendored thirdparty BLAKE3 portable C implementation
• Fixes up LICENSE.txt to properly reference thirdparty code
• Removes unused thirdparty code

[dmbryson]

@swift-ci please smoke test

[dmbryson]

@swift-ci please smoke test windows

[jakepetroules]

@swift-ci please smoke test windows

[dmbryson]

@swift-ci please smoke test windows

[dmbryson]

@swift-ci please test windows

[dmbryson]

@swift-ci please smoke test windows