accept UTF8-BOM when reading dashboard or monitor from JSON file

[kmuto]

Passing a JSON file with a UTF-8 BOM to mkr dashboards push --file-path causes an error that is difficult for users to understand.

     error invalid character 'ï' looking for beginning of value

I understand that "JSON files must not contain a BOM", but I thought it would be more user-friendly to ignore the BOM part and read the file without making an error, so I made a patch.

Test

# original version 0.54.0
$ mkr dashboards push --file-path t-nobom.json
$ mkr dashboards push --file-path t-withbom.json
     error invalid character 'ï' looking for beginning of value

# (remove dashboards)

# patched version
$ ./mkr dashboards push --file-path t-nobom.json
$ ./mkr dashboards push --file-path t-withbom.json

t-nobom.json
t-withbom.json

[lufia]

I do like it a lot!

[lufia]

Could you add a test?

[kmuto]

I added similar BOM handling to monitors code. (it will be called by mkr monitors diff or mkr monitors push)
I added the test unit to monitors/command_test.go also, but gave up to create a test for dashboards because doPushDashboard() is too large to only test BOM.

[lufia]

testing.T.TempDir is useful.

[kmuto]

I had tried it on 8df6538 , 3295eed , and db949f8 but all of them went failed on Windows (I saw golang/go#50510 )
It would be better to use it but I keep CreateTemp code at this time for Windows.