geometry enhancement: ST_GeomFromText() and ST_AsGeoJSON()

[dinglezhang]

Description

I added two geometry sql functions ST_GeomFromText() and ST_AsGeoJSON().
GeoJSON is a popular format in GIS field, so I believe both functions will be very welcome.

Unit test and system test on them are added as well and they are all passed.
Doc will be updated once the changes are accepted.

Checklist

Please check the items in the checklist if applicable.

• Is the user manual updated?
• Are the test cases passed and automated?
• Is there no significant decrease in test coverage?

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