Add ArraySpec

[coretl]

Produce an Spec of points identified by a passed in array. Allow durations to be passed in too.

Array that is passed in should be the midpoints. Upper and lower should be interpolated from it.

This test should be adapted for scanspec and added:
https://github.com/DiamondLightSource/scanpointgenerator/blob/80c1719f69560da715c43ddc9bf458ba55988bdc/tests/test_generators/test_arraygenerator.py#L17-L25

Acceptance Criteria

• Spec exists with tests