feat: add SNS support

[skyrpex]

Basic implementation adding SNS support.

• Resolves SNS support #2
• Needs feat: add SQS support #34

[sam-goodwin]

Are these valid? Are we supporting all possible cases or a subset?

[skyrpex]

Sorry, the PR was meant to be a draft, there's many things I need to clean up first

[skyrpex]

Finally found the format for arrays. It's literally PublishBatchRequestEntries.member.${i + 1}.${memberKey}.

[sam-goodwin]

is there a link to the schema we can include here for reference?

How comprehensive is this passing logic?

[skyrpex]

This is where I look for reference: https://docs.aws.amazon.com/sns/latest/api/API_Operations.html

[skyrpex]

From what I've seen, there's two types of responses. The simple ones (eg, Publish):

<PublishResponse xmlns="https://sns.amazonaws.com/doc/2010-03-31/">
    <PublishResult>
        <MessageId>567910cd-659e-55d4-8ccb-5aaf14679dc0</MessageId>
    </PublishResult>
    <ResponseMetadata>
        <RequestId>d74b8436-ae13-5ab4-a9ff-ce54dfea72a0</RequestId>
    </ResponseMetadata>
</PublishResponse>

Which is most of the actions, and they always look like this:

<{action}Response>
  <{action}Result>
    <{key}>{value}</{key}>
  </{action}Result>
</{action}Response>

But there's also GetTopicAttributes which looks like this:

<GetTopicAttributesResponse xmlns="https://sns.amazonaws.com/doc/2010-03-31/">
    <GetTopicAttributesResult>
        <Attributes>
            <entry>
                <key>Owner</key>
                <value>123456789012</value>
            </entry>
            <entry>
                <key>Policy</key>
                <value>...</value>
            </entry>
            <entry>
                <key>TopicArn</key>
                <value>arn:aws:sns:us-east-2:123456789012:My-Topic</value>
            </entry>
        </Attributes>
    </GetTopicAttributesResult>
    <ResponseMetadata>
        <RequestId>057f074c-33a7-11df-9540-99d0768312d3</RequestId>
    </ResponseMetadata>
</GetTopicAttributesResponse> 

Maybe there's more types, though.

[skyrpex]

Added the following tests:

     ✓ publish
     ✓ publishBatch
     ✓ getTopicAttributes
     ✓ setTopicAttributes
     ✓ subscribe, unsubscribe
     ✓ listTopics

I think all API endpoints will work since it seems pretty consistent.