add pecl source type so we don't get ancient protobuf versions

[henderkes]

What does this PR do?

adds a dedicated pecl source type so we can:

• get the latest version, not just the last one released
• get stable versions instead of beta version (prefer-stable: true, kept this off by default in order to not change existing functionality)

Checklist before merging

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  • Add your test combination to src/globals/test-extensions.php.
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[crazywhalecc]

I've already added on v3 though. Do you think we still need to add new features to v2?

[henderkes]

Only reason I did it is because my CI started being red because of the pecl release of protobuf v3.

[henderkes]

I'll open a new pr instead, let's close this.