Add configure() overload to set the initial LPF state

[davmoli]

Description

LowPassFilter always initializes its state from the first update() input, so there's
no way to start it from a known value.

This PR adds a configure(const T & initial_state) overload that seeds the state: the first
update() returns initial_state and the filter converges towards the input from there.

Fixes #643

Is this user-facing behavior change?

Yes, additive only. Existing configure() calls behave exactly as before.

Did you use Generative AI?

Implementation written by hand. Claude Code (Opus 5) reviewed it and helped write the unit tests.

Additional Information

The new tests cover all three supported types (double, std::vector<double>,
WrenchStamped). These are the first direct unit tests of control_toolbox::LowPassFilter;
the existing ones only reach it through the plugin.

TODOs

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