geo/geogfn: implement ST_AsGeoJSON({geography,int4,int4})

[otan]

Implement ST_AsGeoJSON on arguments {geography,int4,int4}, which should adopt PostGIS behaviour.

Observers: Please react to this issue if you need this functionality.

For Geography builtins, please do the following:

• Ideally add a relevant helper function in pkg/geo/geogfn (parse and output related functions can go in pkg/geo). Add exhaustive unit tests here - you can run through example test cases and make sure that PostGIS and CRDB return the same result within a degree of accuracy (1cm for geography).
• Create a new builtin that references this function in pkg/sql/sem/builtins/geo_builtins.go. Note that we currently do not support optional arguments, so we define functions that have optional arguments once without the optional argument (using the default value in the optional position), and once with the optional argument.
• Modify the tests in pkg/sql/logictest/testdata/logic_test/geospatial to call this functionality at least once. You can call make testbaselogic FILES='geospatial' TESTFLAGS='-rewrite' to regenerate the output. Tests here should just ensure the builtin is linked end to end (your exhaustive unit tests go the above mentioned packages!).
• Ensure the documentation is regenerated by calling make buildshort. You can also play with it by calling ./cockroach demo --empty afterwards.
• Submit your PR - make sure to follow the guidelines from creating your first PR.

You can follow #48529 for an example PR.

_{🤖 This issue was synced with a spreadsheet by gsheets-to-github-issues by otan on 2020-06-30T05:28:55Z. Changes to titles, body and labels may be overwritten.}