feat: Add adjacent_feature_filter to gpf_get_feature

[LionelZoubritzky-IGN]

The problem of #53 was that calling gpf_get_features with typename==XXX and intersects_feature_filter.type==XXX could either mean that:

• the MCP is trying to actually call gpf_get_feature_by_id, or
• the MCP is trying to actually do this query, in order to retrieve the neighboring features.

This PR solves #53 by keeping the current error condition, but also introducing a new adjacent_feature_filter which enables the MCP to actually do the "neighboring features" query.

adjacent_feature_filter only accepts a feature_id and no typename to ensure that the MCP does not use it instead of intersects_feature_filter. It is implemented as a CQL filter with an INTERSECTS condition, doubled by a AND NOT INTERSECTS followed by an inner POINT of the initial geometry. If the initial geometry is made of multiple disjoint sub-geometries, the POINT is replaced by a MULTIPOINT, one per sub-geometry.