Add libcluster_postgres as a third party strategy

[filipecabaco]

Summary of changes

Adding Postgres strategy as a third party strategy

Checklist

• New functions have typespecs, changed functions were updated
• Same for documentation, including moduledocs
• Tests were added or updated to cover changes
• Commits were squashed into a single coherent commit
• Notes added to CHANGELOG file which describe changes at a high-level

[jdewar]

the added line still shows the pasted content of 'Consul clustering strategy' 😬

[filipecabaco]

🤦 copy paste strikes again......... thank you

[nelsonic]

Good addition. No conflicts. Merge?

[hansihe]

Thanks!