[FLINK-11688][checkstyle] Enforce whitespace around TYPE_EXTENSION_AND

[zentol]

Enforce whitespaces around & when used in type definitions, e.g. <T extends T1&T2> would be rejected.

Haven't found a single violation yet, might as well set it in stone. This gets us a step closer to the default checkstyle config, i.e. one special Flink-rule less.

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[tillrohrmann]

Thanks for making our checkstyle stricter @zentol. +1 for merging.

@flinkbot approve all