feat(envelope): Add some useful envelope methods

[dashed]

Add some useful envelope methods needed for integration tests in getsentry/relay#724

TODO

• add some tests

[rhcarvalho]

@dashed I see you want this for getsentry/relay#724, but since the code is not used neither here nor in the Relay PR, it is hard to judge whether it actually helps writing what you're trying to write ;)

One suggestion would be to add simple tests here in sentry-python to showcase the usage (I would not expect someone maintaining this repo to have to read Relay tests).

# t = ...
e = Envelope()
e.add_transaction(t)
with pytest.raises(ValueError):
    e.add_transaction(t)  # can only have 1 transaction in the envelope
assert e.get_transaction() is t

Since there can only be one transaction, perhaps a better API for the present would be a single property -- with the drawback of getting ugly if we ever allow more than 1 transaction per envelope. If that ever happens we can deprecate the property and add add_transaction, perhaps not a big deal.

# t = ...
e = Envelope()
t.transaction = t  # behind the scenes this either adds or replaces an envelope item
assert t.transaction == t

[dashed]

@rhcarvalho I've added some tests 1e29b2a