feat: add config options to DdbToEsSync

[carvantes]

Closes: #96

Functionality for the default deployment remains unchanged

new optional config params:

• esClient: overriding the ES client. Probably easier than having to depend on several env variables
• disableIndexAndAliasCreation: disables index/alias creation.
• getAliasFn: using custom alias/index names

An example using all the optional config values would be:

const ddbToEsSync = new DdbToEsSync({
        esClient: new Client({
            node: 'https://my-search-cluster.us-west-2.es.amazonaws.com',
            Connection: AmazonConnection,
            Transport: AmazonTransport,
        }),
        getAliasFn: (x: any) => ({
            alias: `${x.resourceType.toLowerCase()}-my-test-alias`,
            index: `${x.resourceType.toLowerCase()}-my-test-index`,
        }),
        disableIndexAndAliasCreation: true,
    });

Going forward, We suggest customers to not depend on handleDdbToEsEvent but rather on the new DdbToEsSync class. The original handleDdbToEsEvent was not really meant to be used other than as a lambda handler and is cumbersome to add config options to it without making breaking changes.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.