Graduate the CQL2 modules to supported

[aaime]

It's best if we have OGC API - Features in supported land depending on equally supported foundations.
So, we'll need to graduate the CQL2 modules.

According to procedure, we'll need an e-mail to geotools-devel.
This ticket will track progress, hold a draft of the mail, and track other activities required to achieve supported status.

[aaime]

Email draft:

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Hi all,

I would like to propose moving the cql2-text and cql2-json unsupported modules to supported land.

In particular, the modules support the CQL2 filter language in its text incarnation (similar to ECQL) and its JSON incarnation (for the JSON documents that need to express a filter, and general ease of parsing from machinery (yep, it's 2024 and writing a text parser is still hard).
The modules have been used for a while in the GeoServer OGC API modules, and have been recently overhauled to match the current standard.

The CQL2 specification is modular and allows a subset of all possible filters and expression to be implemented. The CQL2 modules currently support the following requirement classes:

• http://www.opengis.net/spec/cql2/1.0/req/advanced-comparison-operators (like, between, in)
• http://www.opengis.net/spec/cql2/1.0/req/property-property (comparison between two properties)
• http://www.opengis.net/spec/cql2/1.0/req/functions (extensible functions)
• http://www.opengis.net/spec/cql2/1.0/req/basic-spatial-functions (point/bbox/s_intersects)
• http://www.opengis.net/spec/cql2/1.0/req/basic-spatial-functions-plus (all other single and multi geometry types)
• http://www.opengis.net/spec/cql2/1.0/req/spatial-functions (S_CONTAINS, S_CROSSES, S_DISJOINT, S_EQUALS, S_INTERSECTS, S_OVERLAPS, S_TOUCHES, S_WITHIN)

The following are not supported, but may be implemented in the future:

• http://www.opengis.net/spec/cql2/1.0/req/case-insensitive-comparison (CASEI function)
• http://www.opengis.net/spec/cql2/1.0/req/accent-insensitive-comparison (ACCENTI function)
• http://www.opengis.net/spec/cql2/1.0/req/temporal-functions (too many to list, various temporal comparisons between instants and intervals)
• http://www.opengis.net/spec/cql2/1.0/req/array-functions (support for array data types)

In terms of gold stars for graduation:

• Correct headers: check
• Releasable no blocking bugs in jira: check
• Used in anger: sort of, GeoServer OGC API has been using it for a while. Check.
• Optimised: Parsing is as good as it can be. Check.
• Supported: In terms of maintainers, it will be me, Andrea Aime, and Pierre Mauduit (to be confirmed). TBD: some docs

That would be 4+ stars, enough for graduation. In addition to that:

• cql2-text has a test coverage of 74% (when run with -Ponline)
• cql2-json has a test coverage of 70% (when run with -Ponline)
• Both modules pass the QA checks

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To be done IMHO:

• Add a online build running the online tests for these two modules
• Add a small amount of documentation (examples and links to spec)

[aaime]

Documentation available here:
geotools/geotools#4987

The e-mail for graduating the modules has been sent as well.