We need some real extensions in S3 to accurately test the code for
handling remote extensions.
In this PR we just upload three extensions (anon, kq_imcx and postgis), which is
enough for testing purposes for now. In addition to creating and
uploading the extension archives, we must generate a file
`ext_index.json` which specifies important metadata about the
extensions.
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Co-authored-by: Anastasia Lubennikova <anastasia@neon.tech>
Co-authored-by: Alexander Bayandin <alexander@neon.tech>
