Add attributes to XDOM blocks

For different use cases, it is helpful to be able to store attributes on XDOM Blocks. For this, the Block interface should provide methods to get and set them similar to how parameters are handled. This should be implemented in AbstractBlock such that the new properties will be available on all blocks.

These attributes can be used by macros that collect information about other blocks and want to avoid re-computation. For example, the footnote macro will use this to provide the parsed content and unique ids to several putFootnotes-macros as only the last one of them should display the footnotes.

Related forum discussion: https://forum.xwiki.org/t/adding-a-generic-internal-attribute-map-to-xdom-blocks/11788