[link-metrics] split OPENTHREAD_CONFIG_MLE_LINK_METRICS_ENABLE macro

[canisLupus1313]

This commit splits OPENTHREAD_CONFIG_MLE_LINK_METRICS_ENABLE macro into
two subsequent ones:

• OPENTHREAD_CONFIG_MLE_LINK_METRICS_SUBJECT_ENABLE
• OPENTHREAD_CONFIG_MLE_LINK_METRICS_INITIATOR_ENABLE

Signed-off-by: canisLupus1313 85344032+canisLupus1313@users.noreply.github.com

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[edmont]

Thanks. I believe it's safe to remove OPENTHREAD_CONFIG_MLE_LINK_METRICS_INITIATOR_ENABLE from all places in this PR.