Upstream support + example

[aranc]

Added upstream support: https://firebase.google.com/docs/cloud-messaging/receive-upstream

[pep8speaks]

Hello @aranc! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file aiofcm/client.py:

Line 10:80: E501 line too long (89 > 79 characters)
Line 12:80: E501 line too long (90 > 79 characters)

• In the file aiofcm/connection.py:

Line 33:80: E501 line too long (88 > 79 characters)
Line 119:80: E501 line too long (82 > 79 characters)
Line 174:29: E231 missing whitespace after ':'
Line 174:61: E231 missing whitespace after ':'
Line 174:80: E501 line too long (99 > 79 characters)
Line 174:93: E231 missing whitespace after ':'
Line 199:80: E501 line too long (89 > 79 characters)

• In the file examples/upstream.py:

Line 25:22: E261 at least two spaces before inline comment
Line 25:23: E262 inline comment should start with '# '
Line 29:20: E231 missing whitespace after ':'
Line 30:33: E261 at least two spaces before inline comment
Line 44:24: E231 missing whitespace after ':'
Line 45:37: E261 at least two spaces before inline comment

Comment last updated at 2020-12-13 16:51:35 UTC

[aranc]

An issue I encountered: you must send at least one message before firebase starts forwarding you upstream messages. This can be any message, including to invalid device_tokens. Other than that the upsteam seems to work (so far)

[aranc]

There are still similar loss of connection issues. I'll open a new pull request after I'll find the issue

[aranc]

(Moved to: #11)