Added support for LogColorMapper in bokeh Raster plots

[philippjfr]

Adds support for the LogColorMapper via the logz option on Raster based plots.

arr = np.random.rand(10,10)*10000
zero_arr = arr.copy()
zero_arr[5,5] = 0.
hv.Image(arr) + hv.Image(arr)(plot=dict(logz=True))

[jbednar]

Looks good to me; happy to have it merged.

[jlstevens]

Looks good to me too. Merging.

[github-actions]

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