model: implement UnmarshalJSON for CIStr

[tiancaiamao]

What have you changed? (mandatory)

So it can be unmarshaled from a single string

What is the type of the changes? (mandatory)

• Improvement (non-breaking change which is an improvement to an existing feature)

How has this PR been tested? (mandatory)

unit test

Does this PR affect documentation (docs/docs-cn) update? (mandatory)

no

Does this PR affect tidb-ansible update? (mandatory)

no

Does this PR need to be added to the release notes? (mandatory)

no

Refer to a related PR or issue link (optional)

no

Benchmark result if necessary (optional)

I'll add one

Add a few positive/negative examples (optional)

Upgrade from 2.0 to master may meets some problems if partition tables are used.
Because master use

// PartitionDefinition defines a single partition.
type PartitionDefinition struct {
	ID       int64    `json:"id"`
	Name     CIStr    `json:"name"`
	LessThan []string `json:"less_than"`
	Comment  string   `json:"comment,omitempty"`
}

while 2.0 use

// PartitionDefinition defines a single partition.
type PartitionDefinition struct {
	ID       int64    `json:"id"`
	Name     string    `json:"name"`
	LessThan []string `json:"less_than"`
	Comment  string   `json:"comment,omitempty"`
}

After this PR, the upgrade would be smooth.

@winkyao @ciscoxll @shenli

[winkyao]

Add some comment, explain why we need to unmarshal from a single string.

[ciscoxll]

/run-all-tests

[ciscoxll]

LGTM

[winkyao]

LGTM

[coocood]

LGTM

[coocood]

/run-all-tests