Introduce Blob and Blob Loader interface (#3603)
This PR introduces a Blob data type and a Blob loader interface.
This is the first of a sequence of PRs that follows this proposal:
https://github.com/hwchase17/langchain/pull/2833
The primary goals of these abstraction are:
* Decouple content loading from content parsing code.
* Help duplicated content loading code from document loaders.
* Make lazy loading a default for langchain.
