Add support for gRPC-Web text encoding

[emcfarlane]

This adds support for gRPC-Web's text encoding to handle application/grpc-web-text content types. Only client support is added for now, for simplicity. Encoding of request and response body is handled to support text streams. Testing is added by mutating a gRPC-Web client to implement the text protocol.

The implementation is done by converting the request body and response writer to encode and decode streams of base64 encoded data. Go's std library cannot handle padding characters within the stream. For writing base64 encoded data we can simply use base64.NewEncoder, calling Close when needed to flush data to the writer. To read base64 encoded data we cannot use base64.NewDecoder as we need to split the reader on padding characters to form valid base64 chunks to pass to the decoder. Therefore this implementation buffers the stream into 4-byte tokens before decoding, splitting on padding tokens when found.

Closes #143