feat(pose_twist_estimator): automatically initialize pose only with gnss

[shmpwk]

Description

automatic_pose_initializer should be executed only when gnss_enabled is true.
Otherwise, pose_initializer continuously says "GNSS is not supported"
Note that this PR does not affect any system behavior, but if you set gnss_enabled:=false manually, it helps you a lot.

Tests performed

CI

Effects on system behavior

No effect.

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[shmpwk]

One problem of this PR is those who see this pose_twist_estimator launch code for the first time may confuse that why there is such <group if="$(var gnss_enabled)"> even though gnss_enabled is already fixed value.

[YamatoAndo]

I confirmed that when gnss_enabled:=false, the automatic_pose_initializer node does not start, and there is no output saying "GNSS is not supported."

[YamatoAndo]

@isamu-takagi FYI