Export key components of the library

[dkulic]

To facilitate ease of use, key components from the library are exported.

For example, imports:

from didcomm.pack_encrypted import pack_encrypted
from didcomm.unpack import unpack
from didcomm.message import Message, Attachment
# ... etc

Can be simplified as this:

from didcomm import pack_encrypted, unpack, Message, Attachment # ... etc

[dbluhm]

Python convention is to have a list __all__ that is a list of strings of the names of the "exported" components. For example: https://github.com/pydantic/pydantic/blob/main/pydantic/__init__.py#L19

Language servers and other development tools look for this value.