minor change in API to enable non-static usage

[tubbynl]

a smaller change towards #93

this change enables creation of the userContext based on the sessionContext so it can be used outside the static context of BunqContext

the required change is to add the public modifier onto the SessionContext class to expose it's public methods correctly outside the package

[tubbynl]

perhaps the merge target should be master?

[OGKevin]

Some questions/remarks

1. why are you working from master branch ?
2. your manually changing code in generated code, which will be overwritten.
3. Session context should not be publicly accessible, as the end user should not interact with it. hence its package private.
4. The migration to none static is long away, i would like to move everything on one go. Son once we are approaching the 2.0.0 release. there is plenty other issues to tackle before that.

[tubbynl]

1. will fix the branch
2. i need two listing methods for accepting a ApiContext to use; i'll create a Class in .code (like SessionServer) to fix that
3. i don't need Session to be accessible, i only need UserId, and that is currently also exposed via BunqContext.getUserContext().getUserId()
4. These fixes are meant to be non-breaking steps towards the final goal