Support multiple partition keys

[sbstjn]

Currently all events are stored using the same hardcoded partition key:

serverless-analytics/serverless.yml

Lines 194 to 195 in 1e8f9f4

	RequestTemplates:
	application/json: "{\"StreamName\": \"${self:custom.names.kinesis}\",\"PartitionKey\": \"one\",\"Data\": \"$util.base64Encode($input.json('$'))\"}"