rc: fix sizes and durations passed as numbers in flat parameters - #9985

What does this change do?

The rc docs say a size can be passed flat as a number of bytes:

"buffer_size": "42M"
"buffer_size": 44040192

but the second form sets a 42 GiB buffer. configstruct.SetAny turns every flat value into a string and parses it like a command line flag, so a number is read as KiB for a size and as seconds for a duration. The nested _config / vfsOpt form goes through UnmarshalJSON and reads the same numbers as bytes and nanoseconds, which is what the data types section describes.

$ rclone rc --loopback options/local --json '{"buffer_size": 44040192, "timeout": 5000000000}'
master:   "BufferSize": 45097156608, "Timeout": 5000000000000000000
this PR:  "BufferSize": 44040192,    "Timeout": 5000000000

Plain int options were affected too: integers of a million or more come out of fmt.Sprint as 1e+06 and fail to parse.

This decodes numbers as JSON into the field's type, so flat and nested values mean the same thing. Strings still go through the command line parser, so rclone rc ... buffer_size=42M timeout=30s behaves exactly as before, and a number that doesn't decode falls back to the old path.

One behaviour change to be aware of: a duration passed flat as a number was seconds and is now nanoseconds, matching _config and the docs. "timeout": "30s" is unaffected.

Side note: setIfSameType can't match when called from SetAny, since it compares against the type of the any variable rather than the value inside it. I haven't touched it here.

Tests: new TestParseOptions/FlatNumbers subtest. On master it fails with Size:45097156608, Duration:5000000000000000000 and parsing "1e+06" as int failed. make quicktest matches master (cmd/gitannex fails on both, no git-annex here), and golangci-lint run on the changed packages reports 0 issues.

Linked issue

None, the docs already describe the intended behaviour.

Checklist

• This change is trivial OR it has been discussed and agreed in the linked issue.
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• I have added tests for all changes in this PR if appropriate.
• I have added documentation for the changes if appropriate.
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