Add Postgres JSON filters

[henadzit]

Description

This PR:

• Adds new Postgres-specific JSON filter lookup types such as in, not_in, gt, lt, etc. This is based on changes from https://github.com/tortoise/tortoise-orm/pull/1215
• Enables tests from TestJSONFields which were disabled previously by mistake. Apparently if you stack @test.requireCapability(dialect="mysql") and @test.requireCapability(dialect="postgres") on top of each other, the test will not run for neither mysql or postgres
• Fixes JSON array referencing which seems to have been broken for a while, e.g. Model.filter(data__filter={"names_0": "John"})
• Updates the docs

Motivation and Context

JSON and JSONB fields are very popular with people who use Postgres. It makes sense to support a range of JSON operations. Django offers similar functionality.

Related issues:

• will fix https://github.com/tortoise/tortoise-orm/issues/1206
• will fix https://github.com/tortoise/tortoise-orm/issues/1082

How Has This Been Tested?

Added more tests and enabled the existing ones.

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [ ] I have added the changelog accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[coveralls]

Pull Request Test Coverage Report for Build 12016619424

Details

• 28 of 28 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.8%) to 90.282%

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Totals
Change from base Build 12007385729:	0.8%
Covered Lines:	6305
Relevant Lines:	6877

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💛 - Coveralls