Clarify provider conventions for params

The current conventions for parameters is outdated. At some point we
decided that providers can define default values for params. Therefore
parameters must be registered *after* the extension is registered.

Related commit: 9766faf0616a67cd0ff7437950d5453deebac9c4
merged branch igorw/provider-param-convention (PR #622)

This PR was merged into the master branch.

Commits
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3eaa38c Clarify provider conventions for params

Discussion
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Clarify provider conventions for params

The current conventions for parameters is outdated. At some point we
decided that providers can define default values for params. Therefore
parameters must be registered *after* the extension is registered.

Related commit: 9766faf0616a67cd0ff7437950d5453deebac9c4