[ENH] Parameterise the cost function of `dtw` and `adtw` distance measures

[itsdivya1309]

Reference Issues/PRs

Closes: #1874

What does this implement/fix? Explain your changes.

As we discussed on the slack, I've implemented the parameterised DTW and ADTW as private functions to be used in the upcoming Proximity Forest 2.0.

Does your contribution introduce a new dependency? If yes, which one?

Any other comments?

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