Resolves: ClickHouse#51486

Until now, it was illegal to encode 64-bit (unsigned) integers with
MSB=1, i.e. values > (1ULL<<63) - 1, as var-int. In more detail, the
var-int code used by ClickHouse server and client spent at most 9 bytes
per value such that 9 * 7 = 63 bits could be encoded. Some 3rd party
clients (e.g. Rust clickhouse-rs) had the same limitation, whereas other
clients understand the full range (Python clickhouse-driver).

PRs ClickHouse#47608 and ClickHouse#48628 added sanity checks as asserts or exceptions
during var-int encoding on the server side. This was considered okay as
such huge integers so far occurred only during testing (usually fuzzing)
but not in practice.

Issue ClickHouse#51486 is a new fuzzing issue where the exception thrown from the
sanity check led to a half-baked progress packet and as a result, a
logical error / server crash.

The only fix which is not another bandaid is to allow the full range in
var-int coding. Clients will have to allow the full range too, a note
will be added to the changelog. (the alternative was to create another
protocol version but as var-int is used all over the place this was
considered infeasible)

Review note: this is the relevant commit.