Ensure compatibility with stellar-core 15.x

[nebolsin]

stellar-core 15.0.0 introduces breaking changes to the DB schema: stellar/stellar-core#2593

For now I fixed it in a quick and dirty way by applying the following changes to the running stellar-core DB:

CREATE SCHEMA xdrs;
CREATE OR REPLACE FUNCTION xdrs.xdr_int64(xdr bytea, inout pos integer, out result bigint) AS $$
  SELECT pos + 8, ('x' || encode(substring(xdr from pos for 8), 'hex'))::bit(64)::bigint
$$ LANGUAGE SQL IMMUTABLE STRICT;

CREATE OR REPLACE FUNCTION buyingLiabilities(acc accounts) RETURNS bigint AS $$
    SELECT coalesce(buying.value, 0)
    FROM decode(acc.extension, 'base64') accraw(xdrbytes),
         xdrs.xdr_int64(accraw.xdrbytes, 5) as buying(pos, value)
$$ LANGUAGE SQL IMMUTABLE STRICT;

CREATE OR REPLACE FUNCTION sellingLiabilities(acc accounts) RETURNS bigint AS $$
    SELECT coalesce(selling.value, 0)
    FROM decode(acc.extension, 'base64') accraw(xdrbytes),
         xdrs.xdr_int64(accraw.xdrbytes, 13) as selling(pos, value)
$$ LANGUAGE SQL IMMUTABLE STRICT;

CREATE OR REPLACE FUNCTION buyingLiabilities(tl trustlines) RETURNS bigint AS $$
    SELECT coalesce(buying.value, 0)
    FROM decode(tl.extension, 'base64') accraw(xdrbytes),
         xdrs.xdr_int64(accraw.xdrbytes, 5) as buying(pos, value)
$$ LANGUAGE SQL IMMUTABLE STRICT;

CREATE OR REPLACE FUNCTION sellingLiabilities(tl trustlines) RETURNS bigint AS $$
    SELECT coalesce(selling.value, 0)
    FROM decode(tl.extension, 'base64') accraw(xdrbytes),
         xdrs.xdr_int64(accraw.xdrbytes, 13) as selling(pos, value)
$$ LANGUAGE SQL IMMUTABLE STRICT;

It seems to solve an issue for the time being, but we need to update Astrograph ORM layer to work with the new LedgerEntry format which stores extensions as base64-encoded XDR: entry-specific extension (AccountEntryExt, TrustLineEntryExt) in extension column, and common extension (LedgerEntryExt) in ledgerext column. It's similar to how we currently work with account signers.