Make setting the anonymousId public

[gabek]

I'm sure there's a reason why saving the anonymousId isn't made public, but I'm not sure what it is, so I'm submitting this PR if, for nothing else, to re-spur the conversation.

What does this PR do?
Allows setting of the anonymousId in the integrations manager.

How should this be manually tested?

segment.getAnonymousId() -> "abc123"
segment.setAnonymousId("test123")
segment.getAnonymousId() -> "test123"

Validate in the Segment debugger that the correct anonymous Id comes through.

What are the relevant tickets?
#753

Questions:

• Does the docs need an update? Yes
• Are there any security concerns? I'm not aware of any,
• Do we need to update engineering / success? shrug

[f2prateek]

Rather than adding a new method - I think we would want to do what #485 did, and fix the regression in #753.

What happened here was:

1. We merged [Identify] Make Anonymous ID customisable. #485 which allows you to set a custom anonymousId
2. We made some changes that broke the behaviour added in [Identify] Make Anonymous ID customisable. #485 (described in Ability to set anonymousId removed in 3.6.0 #753).

[fathyb]

Fixed with #799