Build endpoint for function definition

[bossenti]

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In https://github.com/apache/streampipes/pull/1147 we introduced the resource for a function definition which defines a registered function in the StreamPipes backend. To provide full support of function API endpoint we need to implement the following aspects:

• [ ] resource container for function definition
• [ ] endpoint for function definition

The REST API endpoint for a function definition is defined here. At least the get() and all() method should be implemented, we might also think about further endpoint methods like create() or delete()

StreamPipes Committer

I acknowledge that I am a maintainer/committer of the Apache StreamPipes project.