Add AWS Firehose provider

[alexdebrie]

What did you implement:

Added AWS Firehose provider.

How did you implement it:

Very similar to the Kinesis implementation from #392

How can we verify it:

1. Create a Firehose stream in AWS.

2. Create a Firehose "function" by calling the registerFunction API. Payload body should be:

{
    "functionId": "firehose-stream",
    "type": "awsfirehose",
    "provider": {
        "region": "<region-of-stream>",
        "awsAccessKeyId": "<access-key-with PutRecord permission>",
        "awsSecretAccessKey": "<secret-access-key>",
        "deliveryStreamName": "<name-of-stream>"
    }
}

3. Create a subscription with the createSubscription API with the following payload:

{
	"functionId": "firehose-stream", 
	"event": "testFirehoseEvent"
}

4. Invoke your event by calling the Events API with a header of event: testFireshoseEvent and a payload body that you wanted inserted to Firehose.

5. Check your Firehose stream destination to make sure your record made it there.

Todos:

• Write tests
• Write documentation
• Fix linting errors
• Make sure code coverage hasn't dropped
• Provide verification commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: NO

[mthenw]

@alexdebrie LGTM, not sure why codecov didn't trigger. I will try to figure out that.

[codecov]

Codecov Report

Merging #394 into master will increase coverage by 0.85%.
The diff coverage is 79.06%.

@@            Coverage Diff             @@
##           master     #394      +/-   ##
==========================================
+ Coverage   58.92%   59.78%   +0.85%     
==========================================
  Files          24       25       +1     
  Lines        1434     1477      +43     
==========================================
+ Hits          845      883      +38     
- Misses        546      551       +5     
  Partials       43       43

Impacted Files	Coverage Δ
providers/awsfirehose/awsfirehose.go	79.06% <79.06%> (ø)
router/router.go	51.32% <0%> (+1.32%)	⬆️

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