Minor: include output partition count of RepartitionExec to tree explain

[2010YOUY01]

Which issue does this PR close?

• Closes #.

Rationale for this change

Now output partition count is displayed inside () after partition scheme like RoundRobin

13)┌─────────────┴─────────────┐
14)│      RepartitionExec      │
15)│    --------------------   │
16)│   input_partition_count:  │
17)│             1             │
18)│                           │
19)│    partitioning_scheme:   │
20)│     RoundRobinBatch(4)    │
21)└─────────────┬─────────────┘

This number's meaning is not obvious, this PR let this output partition count to be displayed separately

13)┌─────────────┴─────────────┐
14)│      RepartitionExec      │
15)│    --------------------   │
16)│   input_partition_count:  │
17)│             1             │
18)│                           │
19)│  output_partition_count:  │
20)│             4             │
21)│                           │
22)│    partitioning_scheme:   │
23)│     RoundRobinBatch(4)    │
24)└─────────────┬─────────────┘

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

[alamb]

Makes sense to me -- thank you @2010YOUY01

One thought i had was to make this even more concise by putting input / output partition counts on a single line

So instead of

39)┌─────────────┴─────────────┐
40)│      RepartitionExec      │
41)│    --------------------   │
42)│   input_partition_count:  │
43)│             1             │
44)│                           │
45)│  output_partition_count:  │
46)│             4             │
47)│                           │
48)│    partitioning_scheme:   │
49)│     RoundRobinBatch(4)    │
50)└─────────────┬─────────────┘

More like

39)┌─────────────┴─────────────┐
40)│      RepartitionExec      │
41)│    --------------------   │
42)│   partition_count(in->out):  │
43)│             1 -> 4             │
48)│    partitioning_scheme:   │
49)│     RoundRobinBatch(4)    │
50)└─────────────┬─────────────┘

[getChan]

could you update docs/source/user-guide/sql/explain.md?
then I will close #15710

[2010YOUY01]

Makes sense to me -- thank you @2010YOUY01

One thought i had was to make this even more concise by putting input / output partition counts on a single line

So instead of

39)┌─────────────┴─────────────┐
40)│      RepartitionExec      │
41)│    --------------------   │
42)│   input_partition_count:  │
43)│             1             │
44)│                           │
45)│  output_partition_count:  │
46)│             4             │
47)│                           │
48)│    partitioning_scheme:   │
49)│     RoundRobinBatch(4)    │
50)└─────────────┬─────────────┘

More like

39)┌─────────────┴─────────────┐
40)│      RepartitionExec      │
41)│    --------------------   │
42)│   partition_count(in->out):  │
43)│             1 -> 4             │
48)│    partitioning_scheme:   │
49)│     RoundRobinBatch(4)    │
50)└─────────────┬─────────────┘

I also prefer this single-line format, updated

[2010YOUY01]

could you update docs/source/user-guide/sql/explain.md? then I will close #15710

I found it's not easy to update (and I don't think it's necessary to keep it in sync)

[alamb]

Thanks @2010YOUY01