aws-c-mqtt: add missing interface definition if shared + modernize more for conan v2

[SpaceIm]

see https://github.com/awslabs/aws-c-mqtt/blob/v0.8.10/include/aws/mqtt/exports.h

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• I've read the contributing guidelines.
• I've used a recent Conan client version close to the currently deployed.
• I've tried at least one configuration locally with the conan-center hook activated.

[SpaceIm]

waiting for #17100, #17104, #17105 & #17107

[CLAassistant]

All committers have signed the CLA.

[ghost]

I detected other pull requests that are modifying aws-c-mqtt/all recipe:

• aws-c-mqtt: add package_type #18199

This message is automatically generated by https://github.com/ericLemanissier/conan-center-conflicting-prs so don't hesitate to report issues/improvements there.

[SpaceIm]

😫

[SpaceIm]

@RubenRBS @uilianries @prince-chrismc could you review please? There are several aws PR to merge (almost 3 months that these PRs have been opened, everything in this ecosystem could have been v2 ready in one day), and they depend on each others. Waiting several weeks each time one is merged (unlocking others) is frustrating (honestly I don't want to contribute anymore), and having to ping is embarrassing.

[AbrilRBS]

Waiting several weeks each time one is merged (unlocking others) is frustrating (honestly I don't want to contribute anymore), and having to ping is embarrassing.

Sorry that you feel this way, just wanted to let you know that whatever your decision is, we appreciate your contributions and the time you have invested in CCI.
As for the review times, you might have noticed that in the last few weeks we have picked up the pace of reviewing, as we have internally implemented some processes to help speed things up without compromising PR quality, which we hope contributors like you are able to see and appreciate as we continue to work hard on it :)

[conan-center-bot]

Conan v1 pipeline ✔️

All green in build 11 (92ebe0c8f7de2de1124f60891215603f388b811b):

• aws-c-mqtt/0.7.12@:
  All packages built successfully! (All logs)

• aws-c-mqtt/0.7.9@:
  All packages built successfully! (All logs)

• aws-c-mqtt/0.7.6@:
  All packages built successfully! (All logs)

• aws-c-mqtt/0.7.10@:
  All packages built successfully! (All logs)

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Conan v2 pipeline ✔️

Note: Conan v2 builds may be required once they are on the v2 ready list

All green in build 6 (92ebe0c8f7de2de1124f60891215603f388b811b):

• aws-c-mqtt/0.7.12@:
  All packages built successfully! (All logs)

• aws-c-mqtt/0.7.10@:
  All packages built successfully! (All logs)

• aws-c-mqtt/0.7.6@:
  All packages built successfully! (All logs)

• aws-c-mqtt/0.7.9@:
  All packages built successfully! (All logs)