Automatically split up each batch into an acceptable request to the Stitch API

[Phanatic]

The Stitch API has a few limitations about the request size and contents :

1. The request includes malformed JSON/Transit, did not provide an array of records, or contains more than 20,000 records.
2. The size of the request body exceeded 20MB.

We do have a batch-size CLI parameter in the ps-http-tap that allows the caller to pick a batch size for the request to Stitch API. But this requires a lot of manual trial and error to get the value right.
Instead of asking the customer to pick a batch size in the request, we will now automatically detect the size of the request body that we send to Stitch API.

Implemented in :

singer-tap/cmd/internal/batch_writer.go

Lines 107 to 112 in fa8a055

	// getBatchMessages accepts a list of import messages
	// and returns a slice of ImportBatch that can be safely uploaded.
	// The rules are:
	// 1. There cannot be more than 20,000 records in the request.
	// 2. The size of the serialized JSON cannot be more than 20 MB.
	func getBatchMessages(messages []ImportMessage, stream *Stream, maxObjectsInBatch int, maxBatchSerializedSize int) []ImportBatch {