Amazon SNS FIFO support

[pik-jonnyl]

Is your feature request related to a problem? Please describe.
The SNS plugin currently can't send to Amazon SNS FIFO topics as an additional parameter - the group ID field - is required. Enabling FIFO compatibility would help negate duplicate messages that can sometimes be produced by SNS.

Describe the solution you'd like
An optional group ID parameter added to the alerta_sns.py plugin script for compatibility with SNS FIFO topics.

Some SNS FIFO documentation:
https://docs.aws.amazon.com/sns/latest/dg/sns-fifo-topics.html