extmod/modbluetooth: Add gap_indicate_service_changed.

[andrewleech]

This function triggers an indication on the GATT service changed characteristic.
The matching change in aioble can be found micropython/micropython-lib#467

Requires extra nimble feature at this stage: apache/mynewt-nimble#1112

I've added some extra notes to the docs to cover some idiosynchacies seen in phone (iOS / android) usage of this feature:

    Note: with an iOS client it's been seen that if this function is called greater than ~100ms 
    after a new connection is established the connection can be dropped, or the characteristics
    covered by the declared range will lock up and be no longer readable, so it's important to 
    send this command within the first few ms after a connection is made.

    Alternatively, it appears that if the start and end handles are both set to ``0x0000``, 
    the lock-up doesn't occur regardless of when the call is made. Android clients appear to 
    accept the null handles as a re-discover all, though the BLE spec makes no mention of this.

[codecov-commenter]

Codecov Report

Merging #8027 (2f11cb3) into master (30b6ce8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #8027   +/-   ##
=======================================
  Coverage   98.47%   98.47%           
=======================================
  Files         153      153           
  Lines       20145    20145           
=======================================
  Hits        19838    19838           
  Misses        307      307           

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[projectgus]

This is an automated heads-up that we've just merged a Pull Request
that removes the STATIC macro from MicroPython's C API.

See #13763

A search suggests this PR might apply the STATIC macro to some C code. If it
does, then next time you rebase the PR (or merge from master) then you should
please replace all the STATIC keywords with static.

Although this is an automated message, feel free to @-reply to me directly if
you have any questions about this.