[idea] Make so_5::agent_t a container for user-specific data

[eao197]

The type so_5::agent_t can be extended with these public methods:

[[nodiscard]] bool
so_user_data_contains(std::type_index data_type) noexcept; // const?

bool
so_user_data_try_add(std::type_index data_type, std::any data_obj);

void
so_user_data_add_or_replace(std::type_index data_type, std::any data_obj);

void
so_user_data_erase(std::type_index data_type) noexcept;

// Removes the value from agent and returns it.
// Throws an exception if values is not found.
[[nodiscard]] std::any
so_user_data_extract(std::type_index data_type);

[[nodiscard]] std::any*
so_user_data_find(std::type_index data_type) noexcept; // const?

This may simplify management of per-agent specific data. Like some run-time monitoring data to be collected for agent by a dispatcher.

Another example: binding of an agent to dispatcher. A disp_binder has to implement preallocate_resources for a new agent. Some disp_binders have to create a new thread for agent and this thread has to be stored somewhere until bind is called. Today it's implemented as a kind of dictionary in a dispatcher. But with user-specific data this thread object can be stored inside the agent itself as a user-data instance. Then it will be extracted in bind method.