Trimmed down version of Abseil for performance critical use cases (#4735)

This PR adds a custom version of Abseil, which is reduced to the bare
minimum to make the following containers work:
* flat_hash_map
* flat_hash_set
* InlinedVector
* Span

GitOrigin-RevId: ae89f4145c5d44fac8361bc00b0d4037bf70f20d

Author: jakra-mb

LICENSE.md

@@ -1761,6 +1761,217 @@ All rights reserved.
 
 ---
 
+### abseil-containers,dev,Apache-2.0,Google,https://github.com/abseil/abseil-cpp.git
+
+```
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+```
+
+---
+
 ### shaderc,v2025.2,Apache-2.0,google,https://github.com/google/shaderc
 
 ```