Add support for custom hash calculation function, allowing content streaming

[idangutman-ms]

This change provides the ability to add a custom function for calculating the content hash.

This is useful for calculating the hash of the content as it is being streamed, instead of requiring the entire content to be loaded into memory, and is necessary for calculating hashes on large files without excessive memory usage.

Example for usage can be seen in test file, where instead of setting p7.Content, we use the p7.HashCalc (setting Content is optional when using HashCalc):

	reader := bytes.NewReader([]byte(longBuffer))
	p7.HashCalc = func(h hash.Hash, b []byte) []byte {
		bufferSize := 128
		buffer := make([]byte, bufferSize)
		for {
			count, err := reader.Read(buffer)
			if err != nil {
				break
			}
			h.Write(buffer[:count])
		}
		return h.Sum(nil)
	}

[jessepeterson]

Hello @idangutman-ms. You may want to check out #73 and #88 and perhaps submit this to https://github.com/smallstep/pkcs7 or another fork. FYI and thanks!

[hslatman]

This has been completed in smallstep#41, and was released in v0.2.0 of https://github.com/smallstep/pkcs7 🎉