Pandas REF: use common utilities when implementing EA._get_common_dtype

From #43409, https://github.com/pandas-dev/pandas/pull/43409#discussion_r703891397.

Look to use is_*_dtype instead of isinstance when implementing _get_common_dtype for EA's.

Comment From: jbrockmendel

is_*_dtype in general is very slow

Comment From: jreback

i think we should fix that (eg there was a fast path pr from joris or tom) i think at some point