SYNPY-1248: add is_synapse_id function to the client

[vpchung]

Fixes #948

Aside from the unit tests, I also did tests with real Synapse entities:

>>> # Not real IDs
>>> syn.is_synapse_id("test")
False
>>> syn.is_synapse_id("123")
False
>>> syn.is_synapse_id("syn123")
False
>>> # Real IDs
>>> syn.is_synapse_id("syn36025126")
True
>>> # Also works when user is logged out or lacks access
>>> syn.logout()
>>> syn.is_synapse_id("syn36025126")
True
>>> # Warning will be given if synID is not a string
>>> syn.is_synapse_id([123])
[WARNING] synID must be a string
False

Note: this should probably be merged after #949, since that PR contains fixes to the workflow.

[thomasyu888]

Thanks for the contribution @vpchung ! Some comments.

[thomasyu888]

🚀 LGTM! Thanks!