Extend multipart upload feature with support for multipart/related requests

[edeckers]

Some background: #135

Currently react-native-blob-courier supports multipart/form-data file uploads only, it should support multipart/related as well.

Proposed solution

• Add a multipartType: "form-data" | "related" field to BlobMultipartMapUploadRequest
• Add a Content-ID field to each part of the request. The value of the Content-ID-field will be the name of the key of the part in the parts dictionary of the upload request, so contents of separate parts can refer to each other through cid's.
• Add inline: boolean = false field to BlobMultipartFormData

Progress

• Add multipartType to BlobMultipartMapUploadRequest
• Add inline field to BlobMultiPartFormData
• Add Content-ID field to each part of the request in Android code base
• Add Content-ID field to each part of the request in iOS code base
• Process inline field in Android code base
• Process inline field in iOS code base
• Add TypeScript tests
• Add Android tests
• Add iOS tests