- calculate ob_key as SHA1(flow_token_secret) if specified - keep old behavior with randomly generated ob_key if not specified

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#### Description <!-- Describe your changes in detail --> Adds ``flow_token_secret``(string) parameter for **outbound** module to give the ability to use persistent ob_key which is used for flow tokens validation.

Useful for cluster configurations with floating IP and ensures successful validation of the previously generated flow tokens after the **kamailio** restart. You can view, comment on, or merge this pull request online at:

https://github.com/kamailio/kamailio/pull/2801

-- Commit Summary --

* outbound: add flow_token_secret param

-- File Changes --

M src/modules/outbound/doc/outbound_admin.xml (22) M src/modules/outbound/outbound_mod.c (22)

-- Patch Links --

https://github.com/kamailio/kamailio/pull/2801.patch https://github.com/kamailio/kamailio/pull/2801.diff