Function provider SPI

[mbali]

The new FunctionProvider Service Provider Interface simplifies the extension of the function set provided by Stencil.

Implementing and registering these new implementations in META-INF/services/io.github.erdos.stencil.functions.FunctionProvider enable other libraries to register their functions automatically.

[codecov]

Codecov Report

Merging #138 (e05ce09) into master (82c07c8) will not change coverage.
The diff coverage is n/a.

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[erdos]

Hey @mbali thanks for the thorough PR and your patience with it :-)

Because the code changed a lot on the master branch since the PR was opened, I reimplemented the functionality in #166 with some small changes:

• The file in META-INF is automatically generated at compile time
• Removed the priority() of function providers. Instead, to keep things simple, functions cannot be redefined with the same name.
• Removed the customer namespace loading (ClojureCallFnProvider) - it is a nice addition to the framework, but it is preferred to adding it when we will have more concrete use cases.

The change goes live in version 0.6.1.

[erdos]

Closing in favour of #166, feature is to be released in version 0.6.1.