fdw: Expose plugin interface to add additional foreign data wrappers

[mfussenegger]

Problem Statement

Currently the foreign data wrappers are statically compiled into CrateDB and adding a new foreign data wrapper requires contributing to CrateDB and having a new release.

Not all data wrappers are in scope of what CrateDB might want to ship per default.

E.g.:

• Integration with hive
• Integration with ES clusters

Possible Solutions

To support such systems CrateDB could expose a plugin interface or SPI

Considered Alternatives

No response