Cellular: Remove friend definitions from cellular state machine

[kivaisan]

Summary of changes

Instead of defining which classes can access state machine, CellularStateMachine
class is now "a normal" CPP class with public API.

Impact of changes

This is potentially a breaking change but as CellularStateMachine is only used internally there shouldn't be any impact.

Migration actions required

None, internal change

Documentation

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Pull request type

[] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[X] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[X] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

@ARMmbed/mbed-os-wan

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[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
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[0xc0170]

@AnttiKauppila Please review. This will be marked as ready for merge once approved.

[adbridge]

NOTE the ci last ran several days ago and thus this will need to be run again before it can be merged, to ensure no conflicts etc due to other merges in the interim!

[0xc0170]

CI restarted

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 2
Build artifacts