filter_state: add new envoy.hashable_string object factory

[agrawroh]

Background

See: https://github.com/envoyproxy/envoy/issues/35711

Changes

This PR adds a new generic string object factory with hashing interface in filter state with key envoy.hashable_string which could be used when unique upstream connection needs to be picked based on the hash of the value of filter state being shared.

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Commit Message: filter_state: add new envoy.hashable_string object factory. Additional Description: See Background Section. Risk Level: Low Testing: Unit Tests Docs Changes: Added the description of the new object factory. Release Notes: Added Platform Specific Features: N/A

Signed-off-by: Rohit Agrawal [email protected]