enhance ee.String.geetools.format with numbers and dates formatters

[fitoprincipe]

In previous versions of batch.Export.imagecollection.toAsset the user was able to choose a format pattern to format the name of the images. This inspired me on enhancing the ee.String.geetools.format method to accept formatters. It adds a bit of complexity by using ee.Algorithms.If twice, but I belive it's not big deal when dealing with strings. Other option could be creating a different method to address this situation.

This is realted to #430.

[12rambau]

No problem for me here I like the approach a lot.

side note, could we align on how Python deals with format in the format function i.e. ":" instead of the "%" sign ?

>>> a = 10.1234
>>> f'{a:.2f}'
'10.12'

[fitoprincipe]

Hm, I am not sure about that. I was planning to use this improved method in ee.batch.Export.imagecollection.toAsset, allowing the user to use any image property in the param namePattern, including system properties like system:time_start, which already contains the proposed character :. For example,

ee.batch.Export.imagecollection.toAsset(
  collection = s2col,
  namePattern: "S2_{system:time_start%tyyyy-MM-dd}",
  assetId = "projects/...."
)

Handling this would involve more conditionals and would be a bit confusing for the user. I'm not 100% sure that % cannot be used as a property name, but it's less common.

Also, I though on being inline with GEE formatters.