kvssink .so with static linkage to cproducer (#1179)

[stefankiesz]

Original PR:
#1179

Description of Changes

(aside from original PR changes)

• Modified the original PR to be backwards compatible.
• When building static libraries, kvssink will remain static as has been in the past, the new build option can force it to be shared when it is enabled.
• Updated ReadMe to reflect these changes.
• Added a new CMake option to the Producer C dependency in this branch to fix a can not be used when making a shared object; recompile with -fPIC build error caused by Curl not using the Position Independent Code compiler flag which prevented a static Curl build from being linked to a shared kvssink build.

Testing

• Confirmed that building with BUILD_STATIC does create an unusable kvssink build that leads to WARNING: erroneous pipeline: no element "kvssink".
• Tested building and running kvssink with the new changes on Ubuntu (static build does not work on Mac) - used the following build command:

cmake .. -DBUILD_STATIC=ON -DBUILD_FORCE_KVSSINK_SHARED=ON -DBUILD_GSTREAMER_PLUGIN=ON ..

• Added a new CI test for this build configuration on Linux.

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[stefankiesz]

ToDO: Change the Producer C dependency version to develop once the corresponding PR is merged.