[ENHANCEMENT] Support curly braces ${} variable syntax in datasource selector shortcut

[celian-garcia]

Description

This PR allows ${myVar} and $myVar syntax when we use variable in some fields.

This is to align with the already supported syntax inside the queries:

---

Here are some examples of usage in the validation schemas:

• Datasource selector being either a datasource variable or a datasource struct

#datasourceSelector: {
	_kind: string
	datasource?: =~#variableSyntaxRegex | {
		kind:  _kind
		name?: string
	}
}

• Prometheus query's minStep being either a duration or a variable

kind: "PrometheusTimeSeriesQuery"
spec: close({
	ds.#selector
	query:             strings.MinRunes(1)
	seriesNameFormat?: string
	minStep?:          =~ds.#durationRegex | =~common.#variableSyntaxRegex
	resolution?:       number
})

I am also planning to use it in the Grafana to Perses migration of datasource with datasource variables. That would fix the migration in case it is defined with ${} syntax in Grafana source dashboard
Pending PR: https://github.com/perses/plugins/pull/775/changes

Screenshots

Checklist

• Pull request has a descriptive title and context useful to a reviewer.
• Pull request title follows the [<catalog_entry>] <commit message> naming convention using one of the
  following catalog_entry values: FEATURE, ENHANCEMENT, BUGFIX, BREAKINGCHANGE, DOC,IGNORE.
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