feat(browser): add user context to MonitoringService

[il-kilo]

Description

Added .userContext object to MonitoringService to be able to provide and persist user.

Motivation and Context

Why is this change required?

• it helps better track user actions, also it reexpose datadog user API, which was hidden before

Checklist

• This is a breaking change:
  • Yes
  • No
• Will this release a new version:
  • Yes
  • No

[skamarakas]

name?: string | undefined; === name?: string same applies for above as well

[il-kilo]

Nice catch. You are right, this is subtle but it doesn't work like I expect it to work...
Even with function tsc is still ok with smth like that, while in fact it should not be.
Like if I mark argument as optional, and function has only one argument then what I strictly expect is that either this argument is not passed (arguments.length = 0) or it is passed and it is user (arguments.length = 0 && arguments[0] instance of User). But ts actually allows passing undefined as first argument, which is not expected IMHO and should not be allowed. WDYT?

[il-kilo]

In other words what I am saying that either as func argument or key in the object there is a way to distinguish between passing value as undefined and not passing value. In object we can use smth like key in object, in function - smth like arguments length. Though, in function you can avoid passing only last arguments anyway, but you still have a way to check this and can count on that...