Move `MetricManifold` to ManifoldsBase.jl and extend its interface

[kellertuer]

I think the metric manifold decorator is mature enough to be moved here,
maybe even with a few metric types (maybe also just EuclideanMetric).

We could use this also to introduce the function

get_metric(::AbstracManifold)

which would return just the .metric field for a MetricManifold and otherwise the default metric (stored in a trait, so maybe a little tricky to obtain, but surely doable).

[mateuszbaran]

Yes, I think basic definitions of MetricManifold could be moved here.

get_metric(::AbstracManifold)

We already have the metric function which does that for MetricManifold. I think we could just extend it to other manifolds.

(stored in a trait, so maybe a little tricky to obtain, but surely doable).

That should be fine if metric/get_metric becomes a trait function.

[kellertuer]

Ah, sure, I forgot about that. Then let's extend that function, sure.